Heat olive oil in an oven-safe skillet (I used a 10 inch cast iron) over medium heat. Add diced bell peppers and red onion. Sauté for 3-4 minutes until vegetables are slightly softened. Season with salt and pepper. Add chopped spinach and tomatoes. Cook for an additional 1-2 minutes until spinach wilts.
Turn heat to low and pour the egg whites evenly over the vegetables in the skillet.
Sprinkle crumbled feta cheese evenly over the top of the egg mixture.
Transfer the skillet to the preheated oven and bake for about 10 minutes, or until the frittata is set.
Once cooked, remove the frittata from the oven and let it cool for a few minutes.
Garnish with chopped fresh parsley before serving.
Slice the frittata into wedges and serve.
Notes
In the fridge: Transfer any leftovers to an airtight container and store in the fridge for up to 3-4 days.
In the freezer: For longer storage, wrap individual portions and freeze for up to 3 months. Thaw overnight in the fridge.
Reheating: You can reheat individual slices in the microwave, oven, toaster oven or air fryer.