I do not know how Nana prepared her frittata however it seems to me my great-aunt Josie baked her in the oven, or at least finished it off in the oven. I make it that way and I also make it as in this recipe, cooked totally in a skillet on the stove top.
Heat about 1/3 cup olive oil in an 8-inch non-stick skillet over medium heat. Add 1/2 pound small zucchini sliced into thin rounds, and cook stirring as you do, until the zucchini soften some and become lightly golden, around 5 minutes.
Add i clove garlic finely minced and stir a couple of turns. Beat 3 tablespoons of pecorino and 2 tablespoons minced parsley into 6 beaten eggs in a large bowl. Season with pepper and salt if you think necessary. I usually find the pecorino has enough saltiness in the dish for my taste.
Remove the zucchini from the skillet with a slotted spoon and stir them into the egg mixture. Return the egg mixture to the skillet and cook over medium heat until the bottom of the frittata is set and golden, around 5 minutes or so.
Put a large plate over the skillet and invert the frittata onto the plate. Slide it back into the skillet to cook the other side, Cook until the bottom is lightly browned, around 4 minutes longer.