In a small saucepan, combine the frozen apple juice, flour, cinnamon and salt. Stir constantly over medium heat until the mixture is thick and bubbly, 3 to 5 minutes. Peel and slice the apples and stir them into the apple juice mixture. Pour the mixture into the unbaked pie shell and dot with butter. Position the top crust over the filling, cutting slits for the steam to escape. Trim and seal the edges. Brush the top very lightly with water and sprinkle with nutmeg if desired. Bake at 450 degrees for 15 minutes. Reduce heat to 350 degrees and bake for 30 minutes more. Serves 6-8