Prep the pan and oven. Heat the oven to 350°F (175°C).
Generously grease a 12-cup muffin tin with cooking spray or a thin film of oil. Make sure the sides and bottom of each cup are coated to prevent sticking.
Chop the mix-ins. Dice the ham and bell pepper, chop the spinach, and finely dice the onion. Keep pieces small so they distribute evenly and bake through.
Whisk the eggs. In a large bowl, whisk the eggs, milk, salt, pepper, and garlic powder until the mixture is smooth and a bit frothy.
This adds lightness to the muffins.
Add the fillings. Stir in the ham, bell pepper, onion, spinach, parsley, and most of the cheese. Reserve a small handful of cheese for topping.
Fill the muffin cups. Divide the mixture evenly among the 12 cups, filling each about 3/4 full. Sprinkle the reserved cheese on top for a golden, melty finish.
Bake. Place the tin on the center rack and bake for 18–22 minutes, until the muffins are set and slightly puffed.
A toothpick inserted in the center should come out clean.
Cool and release. Let the muffins cool in the pan for 5 minutes. Run a thin spatula or butter knife around the edges to loosen, then lift them out.
Serve or store. Enjoy warm, or let them cool completely before refrigerating or freezing. They’ll deflate slightly as they cool—totally normal.