Omelettes. Learn how to make a proper plain omelette, and you can make a number of main dishes just by varying the “innards”– cheese, vegetables, meats, anything. Also, by understanding the omelette, you can dumb it down to simple scrambled eggs or fancy it up to a frittata.
Roast chicken. If you can properly roast a chicken, you can apply the same knowledge to turkey or duck. Thanksgiving dinner won’t be the scary thing it has become to some if you can master a chicken.
Boeuf Bourguignon. Learning to braise a meat in a liquid will set you up to make all kinds of stews and chilies.
Marinara Sauce. Master this and you have mastered all manner of pizza and pasta dishes.
Apple Pie. A tender, flaky pastry can be parlayed into many sweet and savory goodies: summer fruit tarts, pumpkin or sweet potato pie, chicken pot pies, and meat-filled pasties and turnovers.
You don’t need to learn a huge repertoire of dishes. Just take the time to master a few.