First, turn on your oven to 375°F (191°C). Then, get a muffin pan with 12 slots and spray it well with nonstick spray. If you're using muffin liners, spray those too!
In a medium bowl, mix together the eggs, milk, salt, pepper, onion powder, and garlic powder until everything is mixed but not too much – we don't want too much air in there. You'll have about 2 cups of mixture.
Now, let's fill up those muffin cups! Put about 2 tablespoons of chopped bell pepper, spinach, or any other stuff you like into each cup. Then, sprinkle 1 tablespoon of cheese on top of each.
Pour your egg mixture into each muffin cup, filling them about 3/4 full. It might look a bit full, but don't worry – they'll puff up while baking.
Pop the muffin pan into the oven and bake for 18–20 minutes until they're nice and golden brown around the edges. They'll rise up while baking, and if they look a bit uneven, that's totally okay.
Once they're done, take the muffin pan out of the oven and let them cool for about 5 minutes. They'll deflate a bit as they cool, but they'll still taste great!
Finally, take your egg muffins out of the pan and serve them up right away, or let them cool completely and store them in the fridge or freezer. If you're feeling fancy, sprinkle on some optional garnishes before serving.