Preheat oven to 375°F (190°C).
Thaw the frozen spinach and squeeze out every drop of excess moisture using a kitchen towel until very dry.
In a large bowl, whisk together the egg whites, whole egg, and Greek yogurt until smooth.
Stir in the dried spinach, crumbled feta, sliced green onions, chopped dill, parsley, salt, pepper, and nutmeg.
Lightly brush a small baking dish with a tiny bit of olive oil.
Layer the three sheets of phyllo dough into the dish, lightly brushing or spraying between layers, allowing the edges to hang over.
Pour the spinach and egg mixture into the center of the dough.
Fold the overhanging phyllo edges back over the filling to create a rustic border.
Bake for 30-35 minutes until the phyllo is golden brown and the center is set.
Let rest for 5 minutes before slicing and serving.