Finely dice the bell pepper and red onion.
In a medium bowl, whisk together the eggs, liquid egg whites, sea salt, and black pepper until the mixture is uniform and slightly frothy.
Heat the ghee in a 10-inch non-stick skillet over medium heat.
Add the diced bell pepper and red onion to the skillet, sautéing for 3 to 4 minutes until the vegetables are tender.
Add the fresh spinach to the pan and stir for 1 minute until just wilted.
Pour the egg mixture over the vegetables, tilting the pan to ensure the eggs cover the entire surface.
Allow the eggs to cook undisturbed for 2 minutes, then use a spatula to gently lift the edges, letting any uncooked egg run underneath.
Once the top is mostly set, sprinkle the crumbled feta cheese over one half of the omelette.
Carefully fold the omelette in half and cook for an additional 60 seconds to warm the cheese.
Slide the omelette onto a plate and serve immediately with a dollop of Greek yogurt on the side.