Preheat your oven to 375°F. Spray your muffin pan (12-count) with non-stick spray, and spray your muffin liners if you want to use those.
Grab yourself a medium bowl. Whisk your eggs, milk, pepper, garlic powder, onion powder, and salt. Just a quick mix until everything is combined (about 2 cups)
Spoon in your chopped spinach, bell pepper, or whatever you like (2 tablespoons). Sprinkle a generous tablespoon of cheese over the top.
Pour your egg mix into every cup, filling them up to ¾ full. Pop them in the oven. Let them bake for 16 to 20 minutes until they're puffy with the edges turning golden brown. It's normal if they rise unevenly.
Let them sit in the pan (out of the oven) for about 5 minutes. Dig in right away, or let them chill completely and stash them in the fridge or freezer for later. Feel free to go wild with extra garnishes.