Mini Egg-White Muffins

Directions

1. 1
   Preheat the oven to 350 degrees F (175 degrees C). Spray 2 mini muffin tins with nonstick cooking spray.
2. 2
   Heat a medium-sized skillet over medium heat and spray with cooking spray. Add onion and cook gently, 1 to 3 minutes. Add spinach and cook until wilted, about 1 minute more. Remove from heat and allow to cool slightly.
3. 3
   Whisk egg whites and egg together in a bowl. Add Cheddar cheese and season with black pepper. Stir cooled spinach mixture into eggs.
4. 4
   Spoon mixture into each mini muffin cup, filling them halfway.
5. 5
   Bake in the preheated oven until set and puffed up, about 15 minutes.