Preheat oven to 400°.
Over medium-low heat, heat oil in a 10-inch skillet. Stir in onion and cook until slightly brown, about 10 minutes. Add peppers and cook 5 more minutes, then add zucchini. Season with salt and pepper and cook 3 more minutes stirring occasionally.
In a medium bowl whisk eggs, egg whites, parmesan, salt and pepper.
Add eggs to the skillet making sure eggs cover all the vegetables. When the edges begin to set (about 2 minutes) move skillet to oven. Cook about 10-15 minutes, or until frittata is completely cooked.
Serve warm, cut into 4 pieces.