Preheat the oven to 375°F (191°C). Generously spray a 12-cup muffin tin with nonstick cooking spray. Avoid using paper liners, as they can cause the muffins to stick.

In a medium bowl, whisk together the eggs, milk, salt, pepper, onion powder, and garlic powder until well combined. Be careful not to overmix, as this can incorporate too much air into the mixture.

Divide the chopped bell pepper, spinach, and any other desired mix-ins evenly among the 12 muffin cups, filling them about 2/3 full. Sprinkle 1 tablespoon of cheese on top of each.

Carefully pour the egg mixture over the fillings, filling each muffin cup about 3/4 full, just covering the add-ins and cheese.

Bake for 18-20 minutes, until the egg muffins are puffed up and golden brown around the edges. Don't be concerned if the muffins rise unevenly during baking - this is normal.

Remove the muffin tin from the oven and let the egg muffins cool in the pan for 5 minutes. As they cool, the muffins will deflate slightly.

Gently remove the egg muffins from the pan and serve immediately, or allow to cool completely before storing in the refrigerator or freezer.

When ready to serve, you can add any desired garnishes such as chopped parsley, chives, grated Parmesan, or red pepper flakes.