Preheat the oven: Set your oven to 375°F (190°C) and grease a muffin tin with non-stick spray.
Whisk the eggs and milk: In a large bowl, whisk together the eggs and milk 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 bacon, ham, or sausage, stir it in now.
Pour into the muffin tin: Evenly distribute the egg mixture among the muffin cups, filling each about ¾ full.
Bake: Place the muffin tin in the oven and bake for 18-20 minutes, or until the eggs are set and slightly golden on top.
Cool and serve: Let the muffins cool for 5 minutes before removing them from the tin. Serve warm or store for later!