In a medium bowl, whisk together the whole eggs, egg whites, sea salt, and black pepper until the mixture is completely uniform and slightly frothy.
Finely chop the fresh chives and parsley, then set them aside with the crumbled feta, shredded cheddar, and grated parmesan.
Heat a non-stick skillet over medium-low heat and add the ghee, swirling to coat the bottom of the pan.
Add the baby spinach to the pan and sauté for 1 minute until just wilted, then remove the spinach and set aside.
Pour the egg mixture into the same skillet. Use a rubber spatula to gently push the cooked edges toward the center, tilting the pan to let the raw egg flow into the empty spaces.
Once the eggs are mostly set but still slightly moist on top, sprinkle the three cheeses, sautéed spinach, and fresh herbs over one half of the omelet.
Carefully fold the other half of the omelet over the filling and let it sit for 30 seconds to allow the cheese to melt.
Slide the omelet onto a plate and serve immediately with a toasted slice of sourdough bread.