Preheat oven to 375 degrees.
Grease a muffin tin (makes 12 cups).
Place egg whites in a large bowl.
Add milk, while whisking egg whites.
Continue to whisk, until frothy.
Add salt and pepper, to taste.
Add spinach, onion, and then feta.
Stir until mixed evenly.
Full muffin tins with mix.
Bake for 20-23 minutes.
Serve immediately, and enjoy.