In my family, the frittata was a favorite breakfast or brunch dish made with yummy leftover dishes. So the first rule of cooking one is to make a side dish, like sautéed zucchini and tomatoes and to enjoy them with dinner, making sure you cook up enough to have a bowlfull left for the frittata.
As with many rustic recipes, there is no measuring that goes on. You cook with a general sense of proportion and a lot of feel. Zucchini and tomatoes are a favorite combination, flavored with a bit of onion, garlic, and spices, but not too much. The trick is to highlight the flavor of the zucchini and not overpower it. Salt and pepper to taste.
I like to keep the proportion of zucchini and tomato to about 2-1, that is, twice as much zucchini as tomato. In this case, I used halved cherry tomatoes, since they keep their tomato identity well as they sauté. Use about a third as much onion as tomato. This keeps the onion from overpowering the dish. And then the garlic depends on your taste, ranging from an extremely mild small clove of garlic, to two large cloves.
Choose a saucepan that will hold all the ingredients at no more than two layers deep. Over medium heat, sauté the onions and garlic in olive oil. Sprinkle a light dusting of oregano and thyme over the top. If you like the taste of fennel, add some seeds as the browning will bring out their flavor. When the onions are clear, add the tomatoes and zucchini and stir occasionally. Here you want to watch for the change of color in the zucchini from opaque white to the beginnings of a translucent yellow/green. Catch them just as they turn but do not let them cook too long. Once they are glassy through and through, they are also mushy.
Remove and toss with grated parmesan cheese. This is a very quick and tasty side dish for a pasta dinner After dinner, cover the leftovers and place in the refrigerator.
For the frittata, you will need to reheat the mixture first, on the stovetop. In a separate bowl, beat just enough egg to cover the quantity of mixture you have left. Again, there is not wrong way to do it, just go by feel as to whether you want more or less egg remembering that you do need enough to bind the whole panful together. Once the mixture is hot, pour the egg over it and reduce the heat to medium or even low heat if your frittata is particularly big. You want the center to cook and not be runny. Once the mixture begins to look solid, loosen the bottom and shake the pan to slide the frittata onto a large flat plate. If the pan is not too heavy, hold it upside down over the plate, and then, with the pan in one hand and the dish holding the frittata in the other, do a two-handed flip to get it back into the pan on the reverse side. If you are not strong enough do do that, take the plate in two hands, and with a steady confident move - flip the frittata uncooked-side down into the pan and continue cooking until the center is no longer runny.
Serve the frittata with some fresh chopped tomatoes and basil and enjoy.