Don't follow a recipe.
Which is to say, don't just take the recipe as a fixed set of instructions. Compare recipes, look at different ratios of ingredients, get a handle on what happens if you add more fat, less fat, sugar, etc.
Definitely know how to - fry onions without burning them; make a roux; pastry handling; cook potatoes (parboil, full boil, other methods); have a reasonable understanding of the uses of a range of spices and herbs. Oh, and how to chop an onion.
Useful, but probably less relevant to day-to-day cooking, would be learning how to make bread dough and bake bread; make a sponge cake; boil an egg (hard and soft), and - I guess - cook a variety of meats, eg fry a decent steak, bit of fish, etc.