1. Preheat your oven to 350 F (175 C). Lightly grease a 12-cup muffin tin with cooking spray or a little olive oil to prevent sticking.

2. In a large bowl, crack the 12 eggs. Add the milk, salt, and black pepper. Whisk vigorously until the egg mixture is well combined and slightly frothy.

3. Stir in the shredded cheddar cheese, cooked diced ham or turkey sausage, chopped bell peppers, and chopped spinach into the egg mixture. Make sure all the ingredients are evenly distributed throughout the mixture.

4. Carefully pour the egg mixture into each cup of the prepared muffin tin, filling each about two-thirds full. Avoid overfilling, as the eggs will puff up slightly during baking.

5. Place the muffin tin in the preheated oven and bake for 15-20 minutes, or until the egg muffins are set in the center and lightly golden brown around the edges. A toothpick inserted into the center should come out clean.

6. Once baked, remove the muffin tin from the oven. Let the egg muffins cool in the tin for a few minutes before carefully removing them. They should pop out easily if the tin was well-greased. Serve warm or allow them to cool completely for meal prep.