Step 1Heat the olive oil in a 9-or 10-inch nonstick, ovenproof skillet over medium heat. Add the onion, garlic and bell pepper and cook until the onion is tender, about 5 minutes. Add the zucchini and tomato and cook until softened, 5 to 7 minutes.
Step 2Beat the eggs in a large bowl. Beat in the milk and bread crumbs. Stir in the oregano, salt and pepper. Pour this over the vegetable mixture in the skillet and stir to distribute the vegetables evenly. Cover and cook over medium-low heat until the eggs are completely set, about 20 to 25 minutes. Place under the broiler until the top is browned, 2 to 3 minutes. (The frittata can also be served without browning.) Cut into wedges to serve.