Preheat the Oven : Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with cooking spray or a light coat of oil.

Prepare the Vegetables : Chop the onion, tomatoes, bell peppers, and spinach. Make sure all the vegetables are roughly uniform in size for even cooking.

Make the Egg Mixture : In a large mixing bowl, whisk together the eggs, milk (if using), salt, and pepper until well combined. Stir in 1 cup of the shredded cheddar cheese, reserving the remaining 1/2 cup for topping.

Assemble the Casserole : Spread the chopped vegetables evenly across the greased baking dish. Pour the egg mixture over the vegetables, making sure everything is evenly distributed. Sprinkle the remaining 1/2 cup of cheese over the top.

Bake : Bake in the preheated oven for 30–35 minutes, or until the eggs are set and the top is golden brown. To check doneness, insert a knife into the center; if it comes out clean, the casserole is ready.

Serve : Let the casserole cool for 5 minutes before slicing. Serve warm with your favorite sides, like toast, avocado, or fresh fruit.