Preheat the oven to 350 degrees. Grease a 12 cup muffin tin with cooking spray.
In a large bowl, whisk together the eggs, milk, salt and pepper until light and fluffy.
Chop up any vegetables and meats you have and divide them evenly among the 12 cups. Pour the egg mixture into each cup, filling it nearly to the brim. You may have a little egg mixture leftover.
Sprinkle the tops of each cup with your choice of shredded cheese, if desired.
Bake in the preheated oven for 25-30 minutes or until eggs are set and not sloshy at all. Remove from oven and allow to cool slightly before removing. To remove, run a knife along the edge of the cups, and use a spatula to lift them out of the pan.
If refrigerating, allow the cups to cool completely then store in an airtight container for up to 5 days. Reheat in the microwave for 1-2 minutes when ready to eat.