Preheat oven to 350 degrees F. and grease muffin tin with cooking spray.
In a large skillet with some oil, cook the spinach for 2-3 minutes and season to taste.
In a bowl, combine the eggs, milk, and salt.
Add about 1 tbsp of spinach to each muffin cup. Pour the egg mixture into the muffin cups, filling them a little over halfway full. You should end up with 6-7 muffins.
Cover the eggs with cheese and bake for 20 minutes.
Notes
Microwaves and Ovens vary, so adjust time if needed.