Preheat your oven to broil, with a rack high up.
In an 8-inch nonstick oven-safe skillet, melt butter over medium heat.
Sauté red bell pepper until soft, about 6-8 minutes.
Whisk egg whites until frothy in a bowl.
Stir in Italian seasoning, salt, and green onion.
Add spinach slowly, stirring until wilted, about 1-2 minutes.
Gently pour egg whites into the skillet. Cook edges until solid.
Lift edges with a spatula, tilt, and let uncooked whites touch the bottom.
Sprinkle cheese on top, broil for 2-3 minutes until puffed.
Slide onto a plate, cut into wedges, and serve with hot sauce and avocado.