01 - Preheat oven to 350°F. Lightly grease a standard 12-cup muffin tin or fit with silicone liners for easy removal.
02 - In a skillet over medium heat, cook turkey sausage, breaking it up with a spatula, until browned and fully cooked. Drain any rendered fat.
03 - Add red bell pepper and chopped spinach to the skillet. Sauté for 2 to 3 minutes, stirring occasionally, until vegetables are slightly softened. Remove pan from heat and allow mixture to cool slightly.
04 - In a large mixing bowl, thoroughly whisk together eggs, milk, garlic powder, paprika, salt, and pepper until well combined.
05 - Add the cooled sausage and vegetable mixture, green onions, and shredded cheese (if using) to the egg base. Stir gently until evenly distributed. Spoon the mixture evenly into the prepared muffin tin, ensuring each cup is filled about three-quarters full.
06 - Bake in the preheated oven for 20 to 25 minutes, or until egg muffins are risen, fully set, and lightly golden on top.
07 - Remove tray from oven and let muffins cool in the tin for 5 minutes. Carefully loosen edges and transfer to a cooling rack. Serve warm, or cool completely for storage.