Prepare the filling by finely shredding or dicing the pre-cooked chicken breast.
In a small non-stick skillet, melt half of the ghee over medium heat and sauté the sliced mushrooms until they are golden brown.
Add the baby spinach to the skillet and sauté for about 30 seconds until just wilted, then remove the vegetables and set them aside.
In a medium bowl, whisk together the whole eggs, egg whites, sea salt, and black pepper vigorously until the mixture is frothy and well combined.
Wipe the skillet clean, add the remaining ghee, and pour in the egg mixture over medium-low heat.
Using a silicone spatula, gently lift the edges of the omelette as it sets, tilting the pan to allow the uncooked egg to flow underneath.
Once the base is set but the top is still slightly moist, layer the sautéed mushrooms, spinach, shredded chicken, and parmesan cheese over one half of the omelette.
Carefully fold the other half of the omelette over the filling and cook for one more minute until the cheese is melted and the chicken is heated through.
Slide the omelette onto a plate and garnish with freshly chopped chives before serving.