Clean and thinly slice the cremini mushrooms, ensuring they are dry for optimal browning.
Heat the ghee in a 10-inch non-stick skillet over medium heat until shimmering.
Add the mushrooms to the pan and sauté for 5-6 minutes until golden brown and all moisture has evaporated.
Toss in the baby spinach and cook for 1 minute until just wilted, then remove the vegetables from the pan and set aside.
In a medium bowl, vigorously whisk the whole eggs, liquid egg whites, sea salt, and black pepper until well combined and slightly frothy.
Wipe the skillet clean if needed and pour in the egg mixture, reducing the heat to medium-low.
Use a silicone spatula to gently lift the edges of the eggs, allowing the raw liquid to flow underneath until the base is set.
Spread the sautéed mushrooms, spinach, feta cheese, and fresh chives over one half of the omelette.
Carefully fold the omelette in half and slide it onto a plate to serve immediately.