Preheat your oven to 375°F (190°C).
Heat the olive oil in an 8-inch oven-safe skillet over medium heat.
Add the diced yellow onion and sauté for 3-4 minutes until translucent and fragrant.
Stir in the minced garlic and cook for another 30 seconds until aromatic.
Add the baby spinach to the skillet and cook until just wilted, about 1-2 minutes.
In a small bowl, whisk together the liquid egg whites, sea salt, black pepper, and dried oregano.
Pour the egg white mixture over the spinach and onions in the skillet, ensuring they are evenly distributed.
Carefully crack the 3 whole eggs onto the surface of the egg white mixture, spacing them evenly apart.
Sprinkle the crumbled feta cheese over the top of the eggs and spinach.
Transfer the skillet to the oven and bake for 10-12 minutes, or until the egg whites are fully set but the yolks remain slightly runny.
Remove from the oven and let the skillet rest for 2 minutes before serving warm.