Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Heat oil in a frying pan with an ovenproof handle. Sauté onion, garlic, and bell pepper over low heat until tender but not browned.

Add zucchini and cook, stirring occasionally, until tender but still crisp. Add potatoes, stirring frequently, until heated through and slightly sticking to the pan. Stir in chopped tomatoes and black olives and cook until tomatoes release their juices.

Whisk eggs with salt, pepper, oregano, and cayenne; pour over the cooked vegetables.

Arrange tomato slices over the top, sprinkle mozzarella and Parmesan over. Cook gently until eggs are almost set (firm around edges but runny in center).