Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe ramekin or baking dish.
In a skillet over medium heat, add the olive oil and sauté the finely diced yellow onion and minced garlic until translucent and fragrant.
Add the fresh spinach to the skillet in batches, tossing until completely wilted. Transfer the spinach to a clean kitchen towel and squeeze out every bit of excess moisture to ensure the pie isn't soggy.
In a medium bowl, whisk together the large egg, egg whites, and nonfat Greek yogurt until smooth and well-combined.
Stir in the crumbled feta cheese, sea salt, black pepper, nutmeg, and the squeezed-dry sautéed spinach.
Lay one sheet of phyllo dough into the baking dish, allowing the edges to hang over. Lightly mist or brush with a tiny amount of oil, then place the second sheet on top, rotating it slightly to cover more area.
Pour the spinach and egg mixture into the center of the phyllo sheets, then gently fold the overhanging edges of the dough back over the filling.
Bake for 25-30 minutes, or until the egg filling is set and the phyllo crust is beautifully golden brown and crisp.
Allow the pie to cool for 5 minutes before slicing to let the filling firm up.