In a medium bowl, whisk the liquid egg whites, whole egg, sea salt, black pepper, and garlic powder until the mixture is frothy and well combined.
Heat the ghee in a 10-inch non-stick skillet over medium heat, swirling to coat the bottom of the pan evenly.
Add the sliced mushrooms and baby spinach to the skillet, sautéing for 3 to 4 minutes until the mushrooms are tender and the spinach has completely wilted.
Pour the egg mixture over the vegetables and reduce the heat to medium-low, tilting the pan to ensure the eggs reach all the edges.
Cover the skillet with a lid for 2 minutes to allow the top of the omelet to set without overcooking the bottom.
Remove the lid, sprinkle the crumbled feta cheese over one half of the omelet, and carefully fold the other half over the filling using a silicone spatula.
Slide the omelet onto a plate and serve immediately alongside a slice of toasted sprouted grain bread for a complete, clean breakfast.