Preheat oven to 350 degrees F. Grease a muffin tin with olive oil or butter.
In a frying pan, melt the butter and saute the onions and peppers (meat too if using). Once the onions are translucent add the spinach and cook until wilted.
In a large bowl, whisk together eggs, cheese, salt, and pepper. Stir in spinach, onion, and red pepper mixture.
Pour mixture into muffin tin, filling each cup about 3/4 of the way full.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Notes
Note - nutrition calculations are for muffins with only spinach. Meats will ad more.