Heat oil or butter in a non-stick skillet over medium heat. Add spinach and sauté until wilted, about 1–2 minutes. Remove and set aside.
In a bowl, whisk the egg whites with salt, pepper, and garlic powder (if using) until slightly frothy.
Wipe the skillet clean and add a bit more oil if needed. Pour in the egg whites and let cook without stirring until nearly set.
Add sautéed spinach and cheese (if using) to one half of the omelet. Fold the other half over and cook for 1 more minute.
Slide onto a plate and serve hot, garnished with herbs or a splash of lemon if desired.