Preheat the oven: Set your oven to 375°F (190°C) and line a sheet pan with parchment paper or lightly grease it with cooking spray.
Whisk the eggs and milk: In a large bowl, whisk the eggs and milk together until fully combined and slightly frothy.
Add the seasonings: Stir in the salt, pepper, and any additional spices you’d like to use.
Mix in the cheese and veggies: Add the shredded cheese, bell peppers, and spinach to the egg mixture. If using cooked ham, bacon, or sausage, stir it in now.
Pour onto the sheet pan: Evenly distribute the egg mixture onto the prepared pan, making sure all ingredients are spread out.
Bake: Place the sheet pan in the oven and bake for 12-15 minutes, or until the eggs are set and slightly golden on top.
Cool and slice: Let the eggs cool for 5 minutes before slicing them into squares.
Serve and enjoy: Enjoy as-is, or use them in breakfast sandwiches or wraps!