Transfer the store-bought or homemade pie crust to a 9-inch pie plate, gently fit it in, trim any excess dough hanging on the edges, then decorate the edges as desired. Refrigerate it for at least 45 minutes or until you're ready to bake it.
In a medium bowl, mix the brown sugar, then add the flour, nuts in flour, nutmeg, and salt.
Pour the vanilla extract and melted butter over the oat mixture and toss until evenly moistened. Put the streusel in the refrigerator while you prepare the pie filling.
Preheat the oven to 350℉ (177℃).
Peel the apples, then core and slice them ¼-½ inch thick. Place them in a bowl of lemon water to prevent the apples from turning brown while you get the rest of the ingredients together.
Drain the apples and place them in a mixing bowl. Add brown and granulated sugar, cornstarch, cinnamon, nutmeg, ginger, allspice, salt, and lemon juice. Mix well.
Pour the filling into the prepared pie pan, evenly distribute the apples, and arrange the butter pieces over the apples.
Top the apple mixture with the oatmeal crumb mixture and evenly spread it all over the apples.
Place the pie on a baking sheet if your pies tend to spill over in the oven.
Bake the pie until the apples are almost tender when poked with a toothpick, 40-45 minutes. Check the pie a few times throughout baking to ensure the crumb topping and crust aren't over-browning. You may cover it with foil paper if it's browning too fast.
Remove the pie from the oven and let it cool on a wire rack for about 2 hours. Serve warm with vanilla ice cream if desired.