Preheat your oven to 375°F (190°C).
Thinly slice the yellow onion into half-moons.
Heat the avocado oil in a small oven-safe skillet over medium-low heat.
Add the onions and a pinch of salt, cooking for 12-15 minutes until they are soft and deep golden brown.
Stir in the fresh spinach and cook just until wilted, about 1 minute.
In a bowl, whisk together the liquid egg whites, sea salt, and black pepper.
Pour the egg whites over the onion and spinach mixture in the skillet.
Carefully crack the 3 whole eggs on top of the egg white mixture, spacing them evenly.
Transfer the skillet to the oven and bake for 10-12 minutes, or until the whites are set but the yolks are still slightly runny.
Garnish with chopped fresh chives before serving.