Preheat the Oven: Preheat your oven to 350°F (175°C). Generously grease a 12-cup muffin tin with non-stick spray or olive oil. Silicone muffin liners work well to prevent sticking.
Prepare the Egg Mixture: In a large mixing bowl, crack the eggs and whisk them until smooth. Add milk, salt, and pepper to the eggs, whisking again until evenly combined.
Add Mix-Ins: Stir in the shredded cheddar cheese, green onion, and any additional mix-ins like cooked bacon, sausage, or diced vegetables.
Fill the Muffin Cups: Pour the egg mixture evenly into the prepared muffin cups, filling each about three-quarters full.
Bake: Place the muffin tin in the preheated oven and bake for 18–20 minutes, or until the eggs are set and the tops are lightly golden.
Cool and Serve: Allow the muffins to cool in the tin for 5 minutes before removing them. Serve warm, or let them cool completely for meal prep.