Preheat the oven to 350 degrees F. Spray each cup of a muffin pan with one spray of non-sticking cooking spray.
Add 1 heaping tablespoon of the thawed spinach to the bottom of each muffin cup in the muffin tin.
In a medium bowl, whisk together the eggs whites, yogurt, salt and pepper. Evenly divide the egg mixture among the 12 muffin cups. Top each egg muffin with 1 teaspoon of parmesan cheese.
Place in the over and bake for 20-25 minutes or until the eggs are slightly firm to touch. Remove from the oven and set aside to cool for 5 minutes.
Remove from the muffins from the pan and serve or store in an airtight container in the refrigerator for up to one week or freeze and store in a freezer bag for up to 3 months.
To reheat from the refrigerator, place the muffin uncovered on a plate in the microwave for 30 seconds. To reheat from frozen, place the muffin uncovered on a plate in the microwave for 1 minute.