1. Preheat the oven to 375°F (191°C). Spray a 12-count muffin pan generously with nonstick spray. I don’t recommend using muffin liners, but if you’d like to, spray them generously with nonstick spray.
2. In a medium bowl, preferably one with a pour spout, whisk together the eggs, milk, salt, pepper, onion powder, and garlic powder just until combined. Try not to over-mix, or too much air will be incorporated. You will have about 2 cups.
3. Spoon the chopped bell pepper, spinach, and/or other add-ins into each greased muffin cup—about 2 Tablespoons in each. Spoon 1 Tablespoon of cheese on top of each.
4. Pour the egg mixture into each muffin cup, filling about 3/4 full (just over the top of the add-ins and cheese).
5. Bake for 18–20 minutes or until puffy and golden brown around the edges. The muffins rise up tall while baking, and don’t be alarmed if they are rising unevenly—that’s ok.
6. Remove from the oven and cool in the pan for 5 minutes. The muffins deflate as they cool.
7. Remove the egg muffins from the pan, and either serve immediately, or cool completely and store in the refrigerator or freezer (see Notes). Add optional garnishes when serving, if desired.

Recipe source: https://sallysbakingaddiction.com/egg-muffins-recipe/