Preheat the oven to 350°F (175°C). Use cooking spray to lightly grease 12 muffin cups. (You can also use 12 silicone muffin liners.)
Heat a skillet to medium-high heat. Cook the sausage until brown and crumbly. Drain.
Thoroughly beat the eggs in a large bowl. Add in the onions, peppers, salt, pepper, garlic, and cheese. Stir.
Pour the mixture into the greased muffin cups, or silicone liners, until they're about 1/3 of the way full.
Bake for 22-24 minutes or until a toothpick inserted into the center comes out clean.
Notes
STORE Egg Muffin leftovers in an airtight container in the refrigerator for 3-5 days. To FREEZE, wrap each muffin in plastic wrap and place them in a freezer-safe bag for up to 2 months.When you're ready to enjoy them, thaw the muffins if needed, remove the plastic wrap, wrap the muffins in a damp paper towel, and REHEAT them in the microwave until warmed through.