Clean and slice the cremini mushrooms into thin pieces.
In a medium-sized bowl, whisk together the whole eggs, egg whites, sea salt, black pepper, and garlic powder until the mixture is uniform and slightly frothy.
Heat a large non-stick skillet over medium heat and melt the ghee, swirling to coat the surface.
Add the sliced mushrooms to the skillet and sauté for 4-5 minutes until they release their moisture and turn a deep golden brown.
Toss in the baby spinach and cook for 1 minute, stirring constantly until the leaves are just wilted.
Reduce the heat to medium-low and pour the egg mixture into the skillet over the vegetables.
Using a silicone spatula, gently push the eggs from the edges toward the center, allowing the uncooked eggs to flow into the empty space.
When the eggs are about 80% cooked and still look slightly wet, stir in the Greek yogurt and crumbled feta cheese.
Continue to fold gently for another 30 seconds until the yogurt is incorporated and the eggs are set but still moist.
Remove from the heat immediately and serve on a warm plate.