Heat olive oil in a large skillet, then add chopped onion and bell pepper and cook until softened, for about 5-7 minutes. Stir in the mushrooms and cook for another 3-5 minutes until they are browned.

If using cooked breakfast sausage or ham, add it to the skillet with the vegetables and heat through for a couple of minutes.

Grease a 9x13 inch baking dish and spread the cooked vegetable and protein mixture evenly at the bottom.

Whisk together eggs, milk, salt, and pepper in a medium bowl until well combined, then pour this mixture evenly over the vegetable and protein base.

Sprinkle shredded cheese over the casserole and bake in a preheated oven at 375°F (190°C) for 30-35 minutes, or until eggs are set and golden brown. Let it cool for a few minutes before serving.