In a 10 inch cast iron pan, saute the onions until the onions are translucent and soft, Add in the garlic and fry for another 2 minutes. Add in the spinach and cook until the spinach is wilted
In a small bowl, whisk the eggs and egg whites.
Pour over the vegetables in the skillet, stirring if needed to disperse the vegetables evenly.
Place the tomato slices on top. Sprinkle with the feta cheese.
Place in the 350 degree oven and bake for 15-18 minutes, until the egg is fully cooked in the middle of the skillet.
Remove from the oven and serve, using salt and pepper to taste.