Greek Spinach & Feta Pie (Spanakopita)

Greek Spinach & Feta Pie (Spanakopita) For the filling: 2 lb. fresh spinach, washed, dried, trimmed, and coarsely chopped 3 Tbs. extra-virgin olive oil 1 bunch scallions (about 3 oz. or 10 small), white and light-green parts only, trimmed and finely chopped 2 cups crumbled feta cheese (10 oz.) 1/2 cup finely grated Greek kefalotyri cheese or Parmigiano-Reggiano 2 large eggs, lightly beaten 1/2 cup finely chopped fresh dill 1/3 cup finely chopped fresh flat-leaf parsley 1/4 tsp. freshly grated nutmeg Kosher or fine sea salt For the assembly: 1/3 cup extra-virgin olive oil for brushing; more as needed Eighteen 9x14-inch sheets frozen phyllo dough (I use Athens brand), thawed and at room temperature 2 tsp. whole milk