Preheat your oven to 375°F (190°C).
Finely dice the red onion and red bell pepper into uniform pieces.
Heat the olive oil in a 10-inch oven-safe non-stick skillet over medium heat.
Add the onion and bell pepper to the skillet, sautéing for 4-5 minutes until the vegetables are tender and slightly caramelized.
Add the fresh baby spinach to the pan and stir for 1 minute until just wilted.
In a medium mixing bowl, whisk together the egg whites, whole egg, sea salt, black pepper, and garlic powder until well combined.
Pour the egg mixture over the sautéed vegetables in the skillet, tilting the pan to ensure even coverage.
Sprinkle the crumbled feta cheese evenly across the top of the egg mixture.
Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the center is set and the edges are slightly golden.
Remove from the oven and let the frittata rest for 2 minutes before slicing into wedges and serving.