Preheat oven to 375°F (190°C).
Ensure the spinach is completely thawed and squeezed in a kitchen towel to remove all excess moisture.
In a medium skillet, sauté the diced onion and minced garlic in half of the olive oil over medium heat until translucent.
In a large bowl, whisk together the egg, egg whites, Greek yogurt, sea salt, black pepper, and nutmeg.
Stir the squeezed spinach, sautéed onions, crumbled feta, fresh dill, and lemon juice into the egg mixture until well combined.
Lightly brush a small baking dish with a touch of olive oil.
Lay one sheet of phyllo dough into the dish, allowing the edges to overhang, and lightly brush with olive oil.
Place the second sheet of phyllo dough on top, rotating it slightly to cover different angles.
Pour the spinach and egg mixture into the center of the phyllo dough.
Fold the overhanging edges of the phyllo dough over the filling, leaving the center slightly exposed.
Bake for 30-35 minutes until the phyllo is golden brown and the egg filling is set.
Let rest for 5 minutes before slicing and serving.