Preheat oven to 375°F. In a skillet, heat olive oil. Sauté onion, bell peppers, and mushrooms for 5-7 minutes until softened.
In a greased 9x13-inch baking dish, evenly distribute the sautéed vegetables. Scatter the quartered cherry tomatoes on top.
In a large bowl, whisk together eggs, milk, salt, and pepper. Pour this mixture over the vegetables in the baking dish. Sprinkle shredded mozzarella cheese on top.
Bake in the preheated oven for 30-35 minutes, or until the eggs are set and the top is lightly golden. Let it cool for a few minutes before slicing and serving.