Preheat the Oven: Preheat the oven to 375°F (190°C) and grease a 12-cup muffin tin or use silicone liners.
Prepare the Egg Mixture: Whisk eggs in a large mixing bowl, then the heavy cream, salt and pepper until well combined and smooth.
Add Fillings: Whisk in the cheese, spinach, bell peppers and cooked bacon or sausage. Mix well to distribute evenly.
Fill the Muffin Cups: Spoon equal amounts of the mixture into each muffin cup, filling each cup about ¾ full.
Bake: Cook for 18–20 minutes, or until egg muffins are puffed up and center is firm.
Cool and Serve: Cool the muffins for 5 minutes before removing from tin and serving. Store leftovers in the fridge.