Preheat your oven to 375 degrees F. Spray a 12-cup muffin tin generously with nonstick cooking spray.
In a large bowl, whisk together the eggs, milk, salt, and black pepper until smooth and well combined.
Divide the crumbled turkey sausage, shredded cheddar cheese, chopped spinach, diced red bell pepper, and sliced green onions evenly among the 12 muffin cups.
Pour the egg mixture over the fillings in each cup, filling each about three-quarters full. Give each cup a gentle stir with a fork to distribute the ingredients.
Bake for 20 to 22 minutes until the egg muffins are puffed up and set in the center. A toothpick inserted in the middle should come out clean.
Let them cool in the tin for 3 minutes before running a small spoon around the edges and popping them out. Serve warm or store in the fridge for meal prep.