Preheat oven to 350°F (175°C) and grease a 12-cup muffin tin.
In a mixing bowl, whisk together eggs, milk, salt, and pepper.
Add bell peppers, spinach, cheese, and sausage, stirring to combine.
Pour the mixture evenly into the muffin tin cups.
Bake for 20-25 minutes, or until the muffins are set and lightly golden.
Allow to cool slightly before serving or storing in the fridge for up to 4 days.