So a Mind Flayer eats brains as a diet. But can they cook brains into other foods? The book says they need the brains to survive but from what I read the brains don't need to be alive. Does that mean it's possible for them to cook the brains into other foods and eat that and still get the nutrients from the brain? I know it sounds ridiculous but I'm just curious.
MM 221 states,
Illithids subsist on the brains of humanoids. The brains provide enzymes, hormones, and psychic energy necessary for their survival....An illithid experiences euphoria as it devours the brain of a humanoid, along with its memories, personality, and innermost fears. Mind flayers will sometimes harvest a brain rather than devour it, using it as part of some alien experiment or transforming it into an intellect devourer.
This passage suggests that if a brain is not eaten "fresh" from a skull, then it's used for some other non-food purpose. It does not mention saving the brain to be eaten later, cooked or otherwise. Additionally, one could argue that cooking a brain maybe would remove any "psychic energy" it had.
Of course, the DM has full latitude to determine the gastronomy of an illithid--if you're not the DM, you should ask your DM how mind flayers work in their world.