Preheat oven to 375°F (190°C).
Thaw the frozen spinach and squeeze it very dry using a kitchen towel to remove all excess moisture.
In a large bowl, whisk together the egg whites and Greek yogurt until smooth.
Stir in the dried spinach, crumbled feta cheese, sliced green onions, chopped dill, salt, pepper, and nutmeg.
Lightly brush a small baking dish with a tiny bit 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 on top, slightly offset, and brush again.
Pour the spinach mixture into the center of the phyllo sheets.
Fold the overhanging edges of the phyllo back over the filling to create a rustic crust.
Bake for 35-40 minutes until the filling is set and the pastry is golden brown and crispy.