Preheat the oven to 350ºF. Grease a 12-count muffin tin with cooking spray and set aside.In a medium bowl, whisk the eggs until smooth.Add the salt, milk, cheddar cheese, and spinach and mix well.Spoon the mixture into the muffin tins and fill about 2/3 of the way.Top with Parmesan cheese.Bake for 20-22 minutes, or until the edges become golden.