2tablespoonsshredded cheddarfeta, or Parmesan cheese
Hot sauceoptional, for serving
Sliced fresh avocadooptional for serving
Place a rack in the upper third of your oven and preheat the oven to broil.
Heat a small (8-inch) nonstick (it MUST be nonstick) and oven-safe skillet over medium heat. Add the butter. Once the butter is melted, add the red bell pepper. Saute until the pepper softens about 6 to 8 minutes.
Meanwhile, whisk the egg whites in a bowl until they are lightly frothy.
To the skillet, add the Italian seasoning, salt, and green onion. Stir to combine. Let cook 30 seconds.
Add the spinach to the skillet a little at a time, stirring until it wilts and cooks down. This will take 1 to 2 minutes.
Slowly pour the egg whites into the skillet and let cook undisturbed, until the edges begin to cook through and appear more solid white.
With a rubber spatula, gently lift up the edges of the egg whites and tilt the pan so any uncooked whites run underneath the cooked eggs and are now touching the bottom of the skillet. Move your spatula all the way around, lifting the edges and tiling as you go. Let cook 1 more minute.
Sprinkle the cheese on top, then transfer the skillet to the oven. Broil for 2 to 3 minutes (don't walk away!) until the egg whites are puffed and have cooked through. Slide onto a plate or cutting board. Cut into wedges and serve hot, with hot sauce and avocado as desired.
TO STORE: Refrigerate frittata in an airtight storage container for up to 3 days.
TO REHEAT: Rewarm leftovers in a baking dish in the oven at 350 degrees F or reheat gently in the microwave.
TO FREEZE: Freeze frittata in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.