Preheat your oven to 350°F (175°C).
Grease a muffin tin or line it with silicone muffin cups.
In a large bowl, whisk together the eggs and milk.
Add the chopped spinach, diced bell pepper, and cheese to the egg mixture.
Season with salt and pepper to taste.
Pour the egg mixture evenly into the muffin cups, filling each about 3/4 full.
If desired, add cooked bacon or sausage on top.
Bake in the preheated oven for 18-20 minutes, or until the muffins are set.
Remove from the oven and let cool for a few minutes.
Carefully remove the muffins from the tin and enjoy warm or at room temperature.