Heat the oven to 350°F. Spray a 12-cup muffin pan with cooking spray.

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

Stir in the spinach and cheddar.

Pour the mixture evenly into the muffin cups, filling each about three-quarters full.

Bake for 18 to 20 minutes, until the centers are set.

Cool completely before freezing. Freeze in a sealed bag for up to three months.

Reheat frozen muffins in the microwave for 60 to 90 seconds, checking the center before serving.