Finely dice the red bell pepper and red onion.
In a medium bowl, whisk together the large eggs, egg whites, sea salt, and black pepper until well combined and slightly frothy.
Heat the ghee in a non-stick skillet over medium heat.
Sauté the diced peppers and onions for 3-4 minutes until softened and fragrant.
Add the baby spinach to the skillet and cook just until wilted.
Pour the egg mixture over the vegetables, tilting the pan to ensure even coverage.
As the eggs set, gently lift the edges with a spatula to let the uncooked egg flow underneath.
Once the top is mostly set, sprinkle the crumbled feta cheese over one half of the omelet.
Fold the omelet in half and cook for another 30-60 seconds until the cheese is slightly softened.
Slide the omelet onto a plate and serve immediately with a side of plain Greek yogurt.