Heat the oven to 350°F. Spray a 12-cup muffin tin.

Whisk the eggs, milk, salt, and pepper in a bowl.

Divide bacon, cheese, and onion among muffin cups. Pour eggs over them.

Bake for 18 to 22 minutes, until the centers look set.

Cool completely. Freeze in a sealed container for up to 2 months.

Reheat one frozen cup in the microwave for 45 to 60 seconds, checking the center before serving.