Preheat and prep: Preheat your oven to 350°F (175°C). Lightly coat a 12-cup muffin tin with nonstick spray or line with silicone muffin liners.
Cook the turkey sausage: Heat the oil in a skillet over medium heat. Add the turkey sausage, breaking it up with a spatula.
Cook until browned and cooked through, about 5–7 minutes. Drain any excess fat.
Sauté the veggies: In the same skillet, add the onion and bell pepper. Cook 3–4 minutes until softened.
Stir in the chopped spinach and cook just until wilted, 30–60 seconds. Season with a pinch of salt and pepper. Remove from heat and let cool slightly.
Whisk the eggs: In a large bowl, whisk the eggs with milk (if using), garlic powder, smoked paprika, red pepper flakes (if using), 1/2 teaspoon salt, and 1/4 teaspoon black pepper until smooth and well combined.
Combine the fillings: Stir the cooked sausage and sautéed veggies into the egg mixture.
If using cheese, fold in most of it, saving a little to sprinkle on top.
Fill the muffin tin: Divide the mixture evenly among the 12 muffin cups, filling each about 3/4 full. Sprinkle remaining cheese on top.
Bake: Bake for 18–22 minutes, or until the muffins are set in the center and lightly golden on top. A toothpick should come out mostly clean.
Cool and release: Let the muffins cool in the pan for 5 minutes, then run a thin knife around the edges to release.
Transfer to a rack to cool completely if storing.
Serve or store: Enjoy warm, or store for later following the tips below.