Preheat your oven to 375°F (190°C).
In a medium-sized bowl, whisk together the large eggs, liquid egg whites, sea salt, and black pepper until the mixture is well combined and slightly frothy.
Heat the extra virgin olive oil in an 8-inch oven-safe non-stick skillet over medium heat.
Add the diced red onion and red bell pepper to the skillet, sautéing for 4-5 minutes until the vegetables have softened.
Stir in the baby spinach and cook for about 1 minute, just until the leaves have wilted.
Pour the egg mixture into the skillet over the sautéed vegetables, ensuring they are evenly distributed across the pan.
Sprinkle the crumbled feta cheese and chopped fresh chives evenly over the top of the egg mixture.
Allow the frittata to cook on the stovetop for 2-3 minutes without stirring, until the edges are beginning 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.
Remove from the oven and let the frittata rest for 5 minutes before slicing into wedges and serving.