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

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

Divide the bacon, cheese, and onion among the muffin cups. Pour the egg mixture over the top.

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.