Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish or ramekin.
In a large skillet over medium heat, sauté the finely diced yellow onion and minced garlic with a splash of water until translucent and fragrant.
Add the baby spinach to the skillet in batches, stirring constantly until completely wilted.
Transfer the cooked spinach to a fine-mesh colander and use a spatula or clean kitchen towel to squeeze out every drop of excess moisture.
In a medium mixing bowl, whisk together the liquid egg whites, whole eggs, and non-fat Greek yogurt until the mixture is smooth and well-combined.
Fold in the crumbled feta cheese, chopped dill, chopped parsley, sea salt, black pepper, nutmeg, and the squeezed-dry spinach.
Lay one sheet of phyllo dough into the prepared baking dish, allowing the edges to hang over the sides, and lightly brush with half of the olive oil.
Place the second sheet of phyllo on top at a 45-degree angle to the first and brush with the remaining olive oil.
Pour the spinach and egg filling into the center of the dough, then loosely fold the overhanging phyllo edges back over the top of the filling.
Bake for 30-35 minutes until the phyllo is deeply golden and the center of the pie is firm to the touch; let rest for 5 minutes before serving.