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No

Counterspell is used in reaction to the casting, not the effect of the casting.

So in this case at the time Counterspell was used there would be no illusion created (By the time the spell effect shows up Counterspell is too late). The opponent would only have your words and actions to use to interpret what spell is being cast, and those words and actions would be from the illusion spell.

The action economy

There is a rule in XGE that says spell identification takes a reaction, so no single caster can both identify a spell and cast Counterspell in the same turn.

Personal feelings about that being ridiculous aside, that means two people are needed.

Scenario 1

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John identifies the spell and yells to Sarah MsCounterspell that it is just a minor illusion spell

Scenario 2

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John fails to identifie the spell and yells to Sarah MsCounterspell that he doesn't know. At this point Sarah has to decide if it is worth casting Counterspell based on no real knowledge

Scenario 2 is the only way a Counterspell can really be 'tricked'.

In neither scenario does the spell cast by Leon make any difference (Other than to the DC of the identification check), at the time of the identification and the Counterspell there is no effect from the spell, it is still in the process of being cast. It makes no difference if the minor illusion could look exactly like a fireball because it doesn't exist at the point in time of the Counterspell decision.

Counterspell

You attempt to interrupt a creature in the process of casting a spell.

Emphasis mine.

No

Counterspell is used in reaction to the casting, not the effect of the casting.

So in this case, at the time Counterspell was used there would be no illusion created (By the time the spell effect shows up Counterspell is too late). The opponent would only have your words and actions to use to interpret what spell is being cast, and those words and actions would be from the illusion spell.

The action economy

There is a rule in XGE that says spell identification takes a reaction, so no single caster can both identify a spell and cast Counterspell in the same turn.

Personal feelings about that being ridiculous aside, that means two people are needed.

Scenario 1

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John identifies the spell and yells to Sarah MsCounterspell that it is just a minor illusion spell

Scenario 2

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John fails to identifie the spell and yells to Sarah MsCounterspell that he doesn't know. At this point Sarah has to decide if it is worth casting Counterspell based on no real knowledge

Scenario 2 is the only way a Counterspell can really be 'tricked'.

In neither scenario does the spell cast by Leon make any difference (Other than to the DC of the identification check), at the time of the identification and the Counterspell there is no effect from the spell, it is still in the process of being cast. It makes no difference if the minor illusion could look exactly like a fireball because it doesn't exist at the point in time of the Counterspell decision.

Counterspell

You attempt to interrupt a creature in the process of casting a spell.

Emphasis mine.

14:33

No

Counterspell is used in reaction to the casting, not the effect of the casting.

So in this case at the time Counterspell was used there would be no illusion created (By the time the spell effect shows up Counterspell is too late). The opponent would only have your words and actions to use to interpret what spell is being cast, and those words and actions would be from the illusion spell.

The action economy

There is a rule in XGE that says spell identification takes a reaction, so no single caster can both identify a spell and cast Counterspell in the same turn.

Personal feelings about that being ridiculous aside, that means two people are needed.

Scenario 1

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John identifies the spell and yells to Sarah MsCounterspell that it is just a minor illusion spell

Scenario 2

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John fails to identifie the spell and yells to Sarah MsCounterspell that he doesn't know. At this point Sarah has to decide if it is worth casting Counterspell based on no real knowledge

Scenario 2 is the only way a Counterspell can really be 'tricked'.

In neither scenario does the spell cast by Leon make any difference (Other than to the DC of the identification check), at the time of the identification and the Counterspell there is no effect from the spell, it is still in the process of being cast. It makes no difference if the minor illusion could look exactly like a fireball because it doesn't exist at the point in time of the Counterspell decision.

Counterspell

You attempt to interrupt a creature in the process of casting a spell.

Emphasis mine.

No

Counterspell is used in reaction to the casting, not the effect of the casting.

So in this case at the time Counterspell was used there would be no illusion created (By the time the spell effect shows up Counterspell is too late). The opponent would only have your words and actions to use to interpret what spell is being cast, and those words and actions would be from the illusion spell.

The action economy

There is a rule in XGE that says spell identification takes a reaction, so no single caster can both identify a spell and cast Counterspell in the same turn.

Personal feelings about that being ridiculous aside, that means two people are needed.

Scenario 1

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John identifies the spell and yells to Sarah MsCounterspell that it is just a minor illusion spell

Scenario 2

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John fails to identifie the spell and yells to Sarah MsCounterspell that he doesn't know. At this point Sarah has to decide if it is worth casting Counterspell based on no real knowledge

Scenario 2 is the only way a Counterspell can really be 'tricked'.

In neither scenario does the spell cast by Leon make any difference (Other than to the DC of the identification check), at the time of the identification and the Counterspell there is no effect from the spell, it is still in the process of being cast. It makes no difference if the minor illusion could look exactly like a fireball because it doesn't exist at the point in time of the Counterspell decision.

Counterspell

You attempt to interrupt a creature in the process of casting a spell.

Emphasis mine.

14:33

No

Counterspell is used in reaction to the casting, not the effect of the casting.

So in this case at the time Counterspell was used there would be no illusion created (By the time the spell effect shows up Counterspell is too late). The opponent would only have your words and actions to use to interpret what spell is being cast, and those words and actions would be from the illusion spell.

Edit:

The action economy

There is a rule in XGE that says spell identification takes a reaction, so no single caster can both identify a spell and cast Counterspell in the same turn.

Personal feelings about that being ridiculous aside, that means two people are needed.

Scenario 1

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John identifies the spell and yells to Sarah MsCounterspell that it is just a minor illusion spell

Scenario 2

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John fails to identifie the spell and yells to Sarah MsCounterspell that he doesn't know. At this point Sarah has to decide if it is worth casting Counterspell based on no real knowledge

Scenario 2 is the only way a Counterspell can really be 'tricked'.

In neither scenario does the spell cast by Leon make any difference (Other than to the DC of the identification check), at the time of the identification and the Counterspell there is no effect from the spell, it is still in the process of being cast. It makes no difference if the minor illusion could look exactly like a fireball because it doesn't exist at the point in time of the Counterspell decision.

Counterspell

You attempt to interrupt a creature in the process of casting a spell.

Emphasis mine.

14:33

No

Counterspell is used in reaction to the casting, not the effect of the casting.

So in this case at the time Counterspell was used there would be no illusion created (By the time the spell effect shows up Counterspell is too late). The opponent would only have your words and actions to use to interpret what spell is being cast, and those words and actions would be from the illusion spell.

The action economy

There is a rule in XGE that says spell identification takes a reaction, so no single caster can both identify a spell and cast Counterspell in the same turn.

Personal feelings about that being ridiculous aside, that means two people are needed.

Scenario 1

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John identifies the spell and yells to Sarah MsCounterspell that it is just a minor illusion spell

Scenario 2

1. Leon Hexblade starts casting minor illusion
2. John McSpellIdentifier checks to see what spell is being cast. Importantly he does this before any of the spell effects are activated, so just going from the V,S and M components.
3. John fails to identifie the spell and yells to Sarah MsCounterspell that he doesn't know. At this point Sarah has to decide if it is worth casting Counterspell based on no real knowledge

Scenario 2 is the only way a Counterspell can really be 'tricked'.

In neither scenario does the spell cast by Leon make any difference (Other than to the DC of the identification check), at the time of the identification and the Counterspell there is no effect from the spell, it is still in the process of being cast. It makes no difference if the minor illusion could look exactly like a fireball because it doesn't exist at the point in time of the Counterspell decision.

Counterspell

You attempt to interrupt a creature in the process of casting a spell.

Emphasis mine.