Warm Apple Buttermilk Custard Pie

From Cooking Light
The key to both a flaky piecrust and crisp streusel topping is to keep them as cold as possible before putting them into the oven.

Crust:
1/2 (15-ounce) package refrigerated pie dough (such as Pillsbury)
Cooking spray
Streusel:
1/3 cup all purpose flour
1/3 cup packed brown sugar
1/2 teaspoon ground cinnamon
2 1/2 tablespoons chilled butter, cut into small pieces
Filling:
5 cups sliced peeled Granny Smith apple (about 2 pounds)
1 cup granulated sugar, divided
1/2 teaspoon ground cinnamon
2 tablespoons all-purpose flour
1/4 teaspoon salt
3 large eggs
1 3/4 cups fat-free buttermilk
1 teaspoon vanilla extract

Preheat oven to 325º.
To prepare crust, roll dough into a 14-inch circle; fit into a 9-inch deepdish pie plate coated with cookig spray. Fold edges under; flute. Place pie plate in refrigerator until ready to use.
To prepare streusel, lightly spoon 1/3 cup flour into a dry measuring cup; level with a knife. Combine 1/3 cup flour, brown sugar, and 1/2 teaspoon cinnamon in a medium bowl; cut in butter with a pastry blender or 2 knives until the mixture resembles coarse meal. Place streusel in refrigerator.
To prepare the filling, heat a large nonstrick skillet coated with cookig spray over medium heat. Add sliced apple, 1/4 cup granulated sugar, and 1/2 teaspoon cinnamon; cook 10 minutes or until the apple is tender, sitrring mixture occasionally. Spoon the apple mixture into prepared crust.
Combine remaining 3/4 cup granulated sugar, 2 tablespoons flour, salt, and eggs, stirring with a whisk. Stir in buttermilk and vanilla. Pour over apple mixture. Bake at 325º for 30 minutes. Reduce oven temperature to 300º (do not remove pie from oven); sprinkle streusel over pie. Bake at 300º for 40 minutes or until set. Let stand 1 hour before serving. Yield: 10 servings