In a medium skillet, heat the olive oil over medium heat. Add the chopped onion and bell pepper, sautéing for about 3-4 minutes until softened.
Add the chopped spinach to the skillet and cook for another 2 minutes until wilted.
In a mixing bowl, whisk together the egg whites, garlic powder, black pepper, and salt until well combined.
Pour the egg white mixture into the skillet over the sautéed vegetables. Cook for about 3-4 minutes, gently lifting the edges with a spatula to allow uncooked egg whites to flow underneath.
Once the egg whites are mostly set, sprinkle the crumbled feta cheese evenly over one half of the omelet.
Carefully fold the omelet in half over the feta and cook for an additional 1-2 minutes until the cheese is slightly melted and the omelet is fully cooked.
Slide the omelet onto a plate and serve immediately.