Preheat oven to 350°F. Coat an eight-inch oven-safe skillet in cooking spray and place over medium heat. Add onion and zucchini and saute for two to three minutes.
In a large mixing bowl, beat eggs. Add a pinch of salt and pepper and pour over vegetables. Cook until almost set, about six or seven minutes.
Sprinkle cheese on top and transfer to oven. Bake for four to five minutes, or until cheese is melted.