Preheat oven to 350 degrees. Line a 12 cup muffin tin with baking papers.

Heat olive oil in a medium skillet over medium-low heat. Slice leek in half lengthwise, clean and then slice into thin half moons. Add to hot pan, season with a pinch of salt and pepper and cook for about 5 minutes or until soft but not browned.

Squeeze all the liquid out of the thaw spinach and set aside.

In a large mixing bowl, beat egg whites until soft peaks form. In another mixing bowl beat the egg yolks with some salt and pepper until think and creamy looking. Take a big scoop of the egg whites, mix into the yolks and stir well. Fold the yolks into the egg whites. Add the leeks, spinach, nutmeg and cheese and stir well. Spoon mixture into prepared baking pan and cook for 20 minutes or puffed, browned and set.

The frittatas will deflate slightly as they cool.