While the onion is cooking, whisk egg whites with salt and pepper in a medium bowl until frothy. Stir in almost all of the parmesan cheese. Reserve 1 or 2 tablespoons of parmesan cheese for topping.
Once the onion is done, add spinach to the skillet and cook until just wilted, about 30-60 seconds. Spread vegetables evenly around the base of the skillet.
Pour egg white mixture over vegetables and cook over medium heat for 1 minute or until the egg white is just starting to set on the bottom.
Arrange halved cherry tomatoes evenly over the top and sprinkle with remaining parmesan cheese. Cook for another 2 minutes.
Transfer skillet to preheated oven and bake uncovered for 8-10 minutes or until egg is set and the surface is beginning to brown. HINT - if the surface is not browned to your liking, broil (grill) until beginning to brown.
Cut into 4 wedges and serve immediately.
Notes
Variations
Leafy Greens: Swap the spinach for kale or Swiss chard (silverbeet) for a different flavor and texture variation.
Bell Pepper Power: Instead of tomatoes, add red bell peppers (capsicum) for a touch of sweetness and vibrant color.
Cheese Options: Try crumbled feta cheese in place of parmesan cheese for a tangy twist.
Whole Egg Frittata: Want a richer flavor? Make a whole egg frittata by using 4 whole eggs instead of the 8 egg whites called for in the original recipe.
Tips for Success
Use an oven-safe skillet for this egg white frittata. This allows you to seamlessly transfer it from the stovetop to the oven for baking.
A non-stick skillet makes it easier to remove cooked frittata wedges without them sticking.
This recipe is pictured using a 9 ½ inch (24cm) skillet. If you prefer a thicker frittata, choose a smaller skillet. Keep in mind that a smaller skillet will require slightly longer baking time.