Preheat your oven to 350°F (175°C).
Grease a muffin tin with cooking spray or oil.
In a bowl, whisk together the eggs and milk until well combined.
Add salt and pepper to taste.
Stir in the chopped spinach, bell pepper, and cheese.
If using, add cooked bacon or sausage to the mixture.
Pour the egg mixture evenly into the muffin tin, filling each cup about 3/4 full.
Bake in the preheated oven for 20 minutes, or until the eggs are set.
Remove from the oven and let cool slightly before serving.
Enjoy warm or store for later use!