Step 1: Preheat your oven to 350°F (175°C). Prepare a standard 12-cup muffin tin by lightly greasing each cup with cooking spray or lining them with paper muffin liners.
Step 2: In a large bowl, whisk together the 6 large eggs and 1/4 cup of milk until the mixture is light and well combined. Season generously with 1/2 teaspoon of salt, 1/4 teaspoon of black pepper, and 1/4 teaspoon of garlic powder (if using).
Step 3: Next, fold in your chosen fillings: the chopped spinach, diced bell peppers, halved cherry tomatoes, and your preferred cooked protein. Stir until all ingredients are evenly distributed throughout the egg mixture. Lastly, add the shredded cheese and give it one final gentle mix.
Step 4: Carefully spoon the egg mixture into each prepared muffin cup, filling them approximately 3/4 full. Avoid overfilling, as the muffins will puff up during baking.
Step 5: Place the muffin tin into your preheated oven and bake for 18-20 minutes. The muffins are ready when they are set in the center and have a light golden brown color. You can test for doneness by inserting a toothpick; it should come out clean.
Step 6: Once baked, remove the muffin tin from the oven and let it cool for a few minutes before carefully removing the egg muffins. Serve them warm immediately, or allow them to cool completely before storing.