The classic Spanish tortilla is similar to an Italian frittata. You can turn this recipe into a tortilla by simply adding about 300g thickly sliced cold cooked waxy potatoes (such as Charlotte) to the beaten egg with the softened onions.
A frittata is simply a thick, chunky omelette made with some vegetables and usually a little cheese. Served warm, in wedges, it makes an excellent supper. Left to go cold, then sliced into chunks, it’s robust enough to pack into a lunchbox or picnic basket. In fact, it’s ideal prepare-ahead food.
Quite apart from this, frittata appeals to me because you can add almost anything you like to it: bacon or cooked sausage, flaked fish, any kind of cheese, peas or broad beans, asparagus, kale, spinach or broccoli, mushrooms, and so on. My only caveat is to avoid ‘wet’ ingredients such as tomatoes, which will stop the eggs setting properly.