Preheat oven to 350°F.

In a large, nonstick frying pan over medium heat, sauté onion until tender, approximately 5 minutes; remove from heat.

Whisk egg whites in a large mixing bowl.

Thoroughly drain spinach, then add to whites.

Add onion, milk, salt, feta, and cheddar; stir well to combine.

Grease a 9-inch pie pan or a 12-cup muffin pan with nonstick cooking spray.

Pour egg white mixture into pie pan and transfer to oven.

Bake 9-inch pie for 40 minutes and muffins for 15-20 minutes or until whites are set.

Serve immediately.