Warm Apple-Buttermilk Custard Pie

Crust
1/2 (15-ounce) package refrigerated pie dough (such as Pillsbury)
Or
Any basic single pie crust recipe
Streusel
1/3 C. All purpose flour
1/3 C. packed brown sugar
1/2 tsp. ground cinnamon
2 1/2 TB chilled butter cut into small pieces
Filling
5 Cups sliced peeled Granny Smith apple (about 2LBS)
1 C. sugar, divided
1/2 tsp. ground cinnamon
2 TB all purpose flour
1/4 tsp. salt
3 large eggs
1 3/4 C. buttermilk
1 tsp. vanilla extract

1. Preheat oven to 325
2. To prepare crust, roll dough into a 14 inch circle and fit into a 9-inch deep-dish pie plate coated with cooking spray. Fold edges under, and flute. Place pie in refrigerator until ready to use.
3. To prepare streusel, lightly spoon 1/3 C. flour into a dry measuring cup, and level with a knife. Combine 1/3 C. flour, brown sugar, and 1/2 tsp butter with pastry blender until the mixture resembles coarse meal. Place streusel in the refrigerator.
4. To prepare filling, heat a large nonstick skillet coated with cooking spray over medium heat. Add sliced apple, 1/4 C. sugar, and 1/2 tsp. cinnamon; cook 10 minutes or until apple is tender, stirring mixture occasionally. Spoon apple mixture into prepared crust.
5. Combine remaining 3/4 C. sugar, 2 TB flour, salt, and eggs, stirring with a whisk. Stir in buttermilk and vanilla. Pour over the apple mixture. Bake at 375 for 30 minutes. Reduce the oven temp to 300 (do not remove pie from the oven); sprinkle streusel over pie. Bake at 300 for 40 minutes or until set. Let stand 1 hour before serving.