Preheat your oven to 375°F (190°C).
Thaw the frozen spinach completely and squeeze it through a clean kitchen towel until all excess moisture is removed.
In a small skillet, sauté the chopped green onions in the olive oil over medium heat until softened, about 2 to 3 minutes.
In a large mixing bowl, whisk together the egg, egg whites, and Greek yogurt until the mixture is smooth.
Fold in the dried spinach, sautéed green onions, crumbled feta cheese, fresh dill, sea salt, black pepper, and nutmeg.
Lightly coat a small oven-safe baking dish with cooking spray and lay the first phyllo sheet inside, followed by the second sheet, allowing the edges to hang over.
Pour the spinach and egg mixture into the center of the phyllo sheets and fold the overhanging edges back over the filling.
Bake for 30 to 35 minutes until the filling is set and the phyllo crust is deeply golden and crispy.