In a 9-inch non-stick skillet cook the onion, the zucchini, and the peppers with salt and black pepper to taste in 1 tablespoon of the oil over moderate heat, stirring, for 10 minutes, or until the vegetables are tender.
In a bowl whisk together the eggs, 1/3 cup of the Parmesan and the parsley (or cilantro), add the vegetable mixture, and stir the mixture until it is combined well.
In the skillet heat the remaining 1/2 tablespoon oil over moderate heat until it is hot but not smoking, pour in the egg mixture, distributing the vegetables evenly, and cook the frittata, without stirring, for 8 to 10 minutes, or until the edge is set but the center is still soft.
Sprinkle the remaining 1/3 cup Parmesan over the top.
(If the skillet handle is plastic, wrap it in a double thickness of foil.) Broil the frittata under a preheated broiler about 4 inches from the heat for 3 to 4 minutes, or until the cheese is golden, let it cool in the skillet for 5 minutes, and run a knife around the edge.
Slide the frittata onto a serving plate, cut it into wedges, and serve it warm or at room temperature.