Preheat your oven to 375°F (190°C).
In a medium bowl, whisk together the large eggs, liquid egg whites, sea salt, black pepper, and garlic powder until well combined and slightly frothy.
Heat the olive oil in a 10-inch oven-safe non-stick skillet over medium heat.
Add the diced red onion and bell pepper to the skillet, sautéing for 3-4 minutes until the vegetables are tender and fragrant.
Stir in the baby spinach and cook just until wilted, about 1 minute.
Pour the egg mixture over the sautéed vegetables, ensuring they are evenly distributed across the pan.
Sprinkle the crumbled feta cheese over the top of the eggs.
Cook on the stovetop for 2-3 minutes without stirring, until the edges begin to set.
Transfer the skillet to the preheated oven and bake for 10-12 minutes, or until the center is firm and the top is lightly golden.
Let the frittata rest for 5 minutes before slicing into wedges and serving.