Thinly slice the yellow onion into half-moons.
Heat extra virgin olive oil in a non-stick skillet over medium-low heat.
Add the onions and a pinch of salt to the pan, cooking for 10 to 12 minutes until they are soft and golden brown.
In a medium bowl, whisk together the whole eggs, liquid egg whites, sea salt, and black pepper until well combined.
Add the baby spinach to the skillet with the onions and sauté for 1 minute until just wilted.
Pour the egg mixture over the vegetables, tilting the pan to ensure the eggs cover the entire surface.
Cook for 3 to 4 minutes until the edges are set and the center is mostly firm.
Sprinkle the feta cheese over one half of the omelet.
Carefully fold the omelet in half and cook for 1 more minute to allow the cheese to soften.
Slide the omelet onto a plate, garnish with fresh chives, and serve immediately.