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
sub kale or Swiss chard (silverbeet) for spinach
use red bell peppers (capsicum) instead of tomatoes
try feta cheese in place of parmesan cheese
make a whole egg frittata with 4 eggs substituting 8 egg whites
Which skillet to useBe sure to use an oven safe skillet to make this egg white frittata. I suggest you use a non stick skillet so that it is easier to remove wedges.This recipe is pictured in a 9 1/2 inch (24cm) skillet. For a thicker frittata use a smaller skillet. In this case, bake for an extra few minutes.To store leftoversIf you happen to have any frittata leftover, store in an airtight container in the refrigerator because it will spoil easily at room temperature. Eat within 2-3 days.How to reheatEgg white frittata can be reheated in a preheated 350ºF/180ºC oven for 5-10 minutes (depending on the size of leftovers). Cover with foil to prevent drying out.Can frittata be frozen?Frittata freezes quite well depending on the additional ingredients. This frittata could be frozen but the tomatoes will be more watery. If you are substituting the tomatoes with red bell peppers (capsicum), this egg white frittata will freeze well.