Preheat your oven to 375°F (190°C) and lightly grease a 9-inch pie dish or oven-safe skillet with a touch of olive oil.
Heat the olive oil in a medium skillet over medium heat and sauté the diced onion and minced garlic until soft and fragrant.
Add the baby spinach to the skillet and cook for 1-2 minutes until just wilted, then remove from heat and squeeze out any excess moisture.
In a large mixing bowl, vigorously whisk together the whole eggs, egg whites, Greek yogurt, sea salt, black pepper, and nutmeg until the mixture is completely smooth and aerated.
Stir the sautéed spinach mixture and crumbled feta cheese into the egg base until evenly distributed.
Pour the egg mixture into the prepared baking dish, spreading the spinach and feta evenly across the bottom.
Bake for 25 to 30 minutes, or until the edges are golden brown and the center is just set with a slight jiggle.
Allow the quiche to rest for at least 5 minutes before slicing into wedges to ensure the custard sets perfectly.