In a medium bowl, whisk together the eggs, egg whites, sea salt, and black pepper until the mixture is completely uniform and slightly frothy.
Heat a non-stick skillet over medium heat and add the avocado oil, swirling to coat the surface evenly.
Add the diced red onion and red bell pepper to the skillet, sautéing for 3 to 4 minutes until the onions are translucent and the peppers are tender-crisp.
Toss in the baby spinach and cook for about 1 minute, just until the leaves have wilted down.
Pour the egg mixture over the sautéed vegetables, reducing the heat to medium-low to ensure a slow, even cook without browning the bottom too quickly.
Use a silicone spatula to gently lift the edges of the omelet, allowing the raw egg to flow underneath until the base is mostly set.
Sprinkle the crumbled feta cheese over one half of the omelet, then carefully fold the other half over the filling.
Cover the pan with a lid for 30 seconds to allow the feta to soften and the center to finish setting.
Slide the omelet onto a plate and garnish with the freshly snipped chives before serving.