Preheat oven to 325 degrees. Spray a deep dish 9-in pie pan with non stick cooking spray. Line pie pan with crust dough and crimp the edges.

Place in freezer until ready to use.

To make the topping mix together the sugar, flour, oats, and cinnamon. Mix in butter with a fork or stand mixer just until the topping is crumbly. Set aside until ready to use.

To make the filling, melt the butter in a large skillet over medium heat.

Add the apple slices, ¼ cup sugar, and cinnamon. Cook for about 10 minutes, stirring regularly, or until the apple slices are tender. Set aside to cool slightly.

In a bowl, combine remaining ¾ cup granulated sugar, cornstarch, salt, eggs, buttermilk, and vanilla, whisking until smooth.

Pour cooled cooked apples into pie crust then pour buttermilk mixture over apples.

Top pie with streusel topping and cover lightly with foil.

Bake pie at 325º for 30 minutes.

Carefully remove foil from pie and bake for an addition 25-30 minutes or until pie has set.

Let cool for one hour before serving.