Preheat your oven to 375°F (190°C).
In a medium mixing bowl, whisk together the eggs, liquid egg whites, Greek yogurt, sea salt, black pepper, and garlic powder until the mixture is smooth and well combined.
Heat the olive oil in an 8-inch oven-safe non-stick skillet over medium heat.
Add the diced red onion and chopped red bell pepper to the skillet, sautéing for about 4-5 minutes until the vegetables are tender.
Add the baby spinach to the skillet and stir for 1 minute until it just begins to wilt.
Pour the egg mixture over the sautéed vegetables in the skillet, ensuring they are evenly distributed.
Sprinkle the crumbled feta cheese over the top of the egg mixture.
Allow the frittata to cook on the stovetop for 2-3 minutes without stirring, just until the edges begin to set.
Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the center is set and the top is lightly golden.
Remove from the oven, let it cool for a few minutes, then slice and serve.