Preheat the oven to 350°F (175°C).

In an oven-safe skillet, heat the olive oil over medium heat.

Add the diced onions and bell peppers, cooking until softened, about 5 minutes.

Stir in the chopped spinach and cook until wilted, around 2 minutes.

In a mixing bowl, whisk the egg whites until frothy.

Pour the whisked egg whites over the sautéed vegetables in the skillet.

Gently stir to combine, ensuring even distribution.

Add salt, pepper, and any optional herbs to taste.

Sprinkle the shredded cheese on top of the egg mixture.

Transfer the skillet to the preheated oven and bake for about 15-20 minutes, until the center is set and slightly golden.