Make the filling: in a medium pot, melt the butter over medium heat. Cook the butter, swirling occasionally, until the milk solids begin to brown, and butter smells toasty, 4-6 minutes.

Add the apples, sugar, boiled cider (if using), and lemon juice, and stir well to coat. Continue to cook over medium heat, stirring occasionally, until the sugar begins to melt and more fully coat the apples and the apples will begin to release their liquid, 4-5 minutes.

Add the cinnamon, ginger, nutmeg, and salt, and stir well to combine. Continue to cook for 2 minutes more.

Meanwhile, whisk the granulated sugar and flour together to combine. Sprinkle this mixture over the surface of the pot, and mix well to combine. Continue to cook, stirring constantly, until the mixture begins to noticeably thicken, 2-3 minutes. Reduce heat to medium-low, and continue to cook until the mixture begins to form fat bubbles on the surface, 3-4 minutes more.

Stir in the vanilla paste or extract, and transfer the mixture to a heat-safe container to cool completely to room temperature. Or, to cool quickly, pour onto a baking sheet or large baking dish - the increased surface area will help it cool faster. Keep it covered while it cools to prevent it drying out. The filling can be made up to 2 days ahead. Store in the refrigerator, and bring to room temperature before assembling the pie.

Pour the cooled filling into the baked and cooled par-baked crust, and spread into an even layer. Set on a parchment-lined baking sheet. Preheat the oven to 375°F / 190°C with the oven racks in the upper and lower thirds of the oven.

Make the crumb topping: in a medium bowl stir the oats, all purpose flour, whole wheat flour, brown sugar, baking powder, cinnamon, and salt to combine. Rub the butter into the flour mixture with your fingers until the mixture forms large clumps.

Sprinkle the topping evenly over the surface of the pie. Transfer the pie on the baking sheet to the lower oven rack, and bake until the streusel is browned, and the filling bubbles vigorously around the outer edges, 45-50 minutes. If additional browning is needed, transfer the pie to the upper oven rack during the last 10-15 minutes of baking. If the crust is browning too quickly, you can tent the edges with foil for the remainder of bake time.

Cool the baked pie completely to room temperature before slicing. If you want to serve the pie warm, refresh the cooled pie by tenting it in foil and transferring it to the oven on the lower rack. Turn the oven on to 375°F / 190°C. Once the preheat bell dings, set a timer for 10 minutes. Then remove the pie from the oven, unwrap, and slice it. If using, dust the surface of the pie with powdered sugar just before serving.