.Place the mushrooms in a microwave safe bowl with a teaspoon of water and microwave on high for 2-3 minutes, until they have shrunk and softened up. Drain the water and place to the side
.Fry the onions and garlic in a cast iron skillet (or an oven proof skillet of any kind) until translucent and soft.
Add in the spinach and cook until wilted.
Whisk together the eggs and egg whites until combined then mix in the mushrooms. Pour over the onion mixture in the skillet.
Crumble the cubes of feta over the top. (I like to leave some whole as well, I love feta!)
Place the skillet in the preheated oven and bake for 15 minutes, until the eggs are completely cooked in the center of the skillet.