**Preheat Oven:** Preheat your oven to 375°F (190°C) and spray a muffin tin with non-stick cooking spray.
**Prepare Egg Mixture:** In a mixing bowl, whisk together the eggs, milk, salt, and pepper until well combined.
**Add Fillings:** Evenly distribute the bell peppers, onions, spinach, cheese, and optional meat into the muffin tin cups.
**Pour Egg Mixture:** Carefully pour the egg mixture over the fillings in each cup, filling about 3/4 full.
**Bake:** Place the muffin tin in the oven and bake for 20–25 minutes, or until the egg muffins are set and lightly golden on top.
**Cool and Serve:** Let the muffins cool in the pan for 5 minutes before removing. Serve warm or store for later.