Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe skillet or 6-inch baking dish.
Heat the olive oil in a pan over medium heat, then add the diced onion and minced garlic, sautéing until fragrant and translucent.
Add the fresh spinach to the pan and cook until completely wilted, then remove from heat and let cool slightly.
In a medium bowl, whisk together the liquid egg whites, whole egg, and Greek yogurt until smooth and well combined.
Stir the sautéed spinach mixture, crumbled feta cheese, sea salt, black pepper, and nutmeg into the egg mixture.
Lay one sheet of phyllo dough into the prepared baking dish, allowing the edges to hang over, then top with the second sheet.
Pour the egg and spinach filling into the center of the phyllo dough and fold the overhanging edges inward to create a rustic crust.
Bake for 25 to 30 minutes, or until the filling is set and the phyllo crust is golden brown and crispy.