Preheat oven to 350 °F.
Spray nonstick 12-cup muffin tin with nonstick cooking spray to coat the entire tin.
Mix the spinach and ham in a bowl and divide the mixture between the 12 muffin cups.
Mix the eggs, milk, salt, pepper, and cheese together in a bowl.
Take the egg mixture and pour over the spinach and ham in each of the 12 muffin cups. Fill each muffin cup almost to the top.
Bake for about 20 minutes, or until a toothpick or knife can be inserted in the center of the frittata muffin and come out clean.
Cool for 5 to 10 minutes. When ready, slide a knife around the inside of each frittata muffin cup to loosen.
Remove frittata muffins from the muffin cups.