In a large bowl, mix together flour, brown sugar, butter and oats with a pastry blender. Set aside one cup of mixture for crumb topping.
Press the remaining crumb mixture into an ungreased 9-inch pie pan; set aside.
In a sauce pan, combine sugar, cornstarch and water over medium-high heat.
Bring to a boil and let boil for 2 minutes, whisking constantly (it will thicken up quickly so it's important to keep stirring the whole time!).
Remove from heat and fold in apples, vanilla, cinnamon, nutmeg, ginger, and allspice.
Pour apple mixture into prepared crust and sprinkle seet-aside crumb topping on top.
Bake for 40-50 minutes, or until topping is golden brown and apples are tender.
Let cool before serving.
Notes
We recommend using Granny Smith apples for this recipe. They are tart, tangy, and perfectly firm. If you are looking for a sweeter apple, try Honeycrisp, Braeburn, or Jonathan apples. This pie can be made a day in advanced, and it still tastes amazing.