Preheat the oven to 375 degrees F (190 degrees C).

Sift flour, brown sugar, ginger, cinnamon, baking powder, salt, and allspice together in a large bowl. Cut in butter using your fingers, a pastry cutter, or a food processor until the crumbs resemble cornmeal. Drizzle water and molasses over crumbs, mixing with fingers, until the crumbs begin to cling together. Gather the dough and knead it into a ball. Press crust evenly into the bottom and up the sides of a pie pan.

Bake in the preheated oven until lightly golden, 10 to 15 minutes. Remove crust from oven and reduce oven temperature to 350 degrees F (175 degrees C).

Combine pumpkin, dark brown sugar, cinnamon, cornstarch, ginger, salt, nutmeg, allspice, and cloves in a large mixing bowl. Pour in cream, eggs, syrup, and vanilla extract and whisk to combine. Pour maple-pumpkin filling into the baked pie crust.

Bake in the lower third of the hot oven until center of the pie is almost set, 50 minutes to 1 hour.

Meanwhile, combine brown sugar, flour, butter, and cinnamon in a food processor. Pulse until crumbly. Stir in chopped walnuts by hand. Set walnut crumble aside.

Remove pie from the oven and top with walnut crumble. Return pie to the hot oven and continue baking until crisped, about 10 minutes more. Remove pie from oven and turn broiler on.

Bring maple syrup to a boil in a small, heavy-bottomed saucepan over medium heat, about 5 minutes. Clip an instant-read thermometer onto the pan and watch for the syrup to reach 225 degrees F (110 degrees C).

Place egg whites into the bowl of a stand mixer fitted with the whip attachment. Start whipping at medium-high speed. Slowly sprinkle sugar into the whites and whip until soft peaks form, about 2 minutes.

Check the syrup and when the temperature reaches 238 degrees F (114 degrees C), reduce the mixer speed to medium and carefully add the hot maple syrup by pouring it along the side of the bowl. Increase mixer speed to medium-high and continue whipping until meringue is thick and shiny, an additional 3 to 4 minutes.

Use a spatula to spread meringue over the entire pie all the way to the crust. Toast meringue under the hot broiler until lightly brown, 1 to 2 minutes.

Serve chilled or at room temperature.