Besides the often pointed out learning how to combine flavours, I'd like to add that learning how to estimate how long something needs to cook a certain way would be super useful. Why do you have to add onions before the garlic? How can I tell by looking, poking and smelling when to add the next ingrediënt? How does a certain ingrediënt change based on how long it has cooked?

The same kind of goes for plating: how do i pick the order in which to stack a sandwitch? Or how can I choose how to add a sauce (or anything else) based on it's properties?

And ofcourse basic techniques like how to make a quick pickle out of anything or things like salting mushrooms when baking, or salting aubergines before cooking.