Preheat your oven to 375°F (190°C).
Melt the ghee in a 10-inch oven-safe cast iron skillet over medium-low heat.
Add the thinly sliced yellow onion and a pinch of the sea salt, cooking slowly for 15-20 minutes until deep golden brown and caramelized.
Add the fresh baby spinach to the skillet and sauté for 1-2 minutes until just wilted, then spread the vegetables evenly across the bottom.
In a medium bowl, whisk together the eggs, egg whites, Greek yogurt, sea salt, black pepper, and garlic powder until completely smooth and aerated.
Pour the egg mixture over the vegetables in the skillet, ensuring they are well-distributed.
Sprinkle the crumbled feta cheese evenly over the top of the eggs.
Transfer the skillet to the oven and bake for 12-15 minutes, or until the center is set and the edges are slightly puffed.
Let the frittata rest for 5 minutes before slicing into wedges and serving.