Prepare a 9-inch springform pan or square baking dish by lining it with parchment paper for easy frittata removal.
In a mixing bowl, whisk together eggs, mozzarella cheese, milk, spinach, bell peppers, onions, salt, pepper, and Italian herbs until well combined.
Transfer the egg mixture into the prepared pan, spreading it evenly.
Place the pan in the oven and bake for about 25 minutes, or until the frittata is golden, puffy, and the center is firm to the touch.