Place ginger root and ¼ cup sugar in food processor bowl with knife blade attachment; pulse until ginger is finely minced then add ¼ cup flour and pulse a couple of times. Remove 3 tablespoons of this ginger paste* to use in pastry.
Add brown sugar and ½ cup flour to remaining ginger paste and pulse to blend.
Add ¼ cup butter pieces. Pulse until mixture resembles coarse crumbs.
Remove from processor bowl, stir in walnuts and set aside.
Combine flour and salt in food processor with knife blade attachment and process for about 10 seconds.
Add cold butter cubes; process by pulsing 8 to 10 times or until most of the butter is about pea-sized. Do not over mix.
Add reserved ginger paste.
Add water through food chute with processor running. Add only enough water to moisten so dough begins to clump.
Form into a 4-5 inch disk, place in a food storage bag and refrigerate for 30 minutes before rolling.
Roll out dough on a lightly floured surface, into a 12” round; fit pastry into a 9-inch pie pan. Freeze crust before filling.
Preheat oven to 425°F.
Wisk together all ingredients until well blended.
Pour into frozen prepared pie shell.
Bake 20 minutes. Reduce temperature to 350°F; bake 20 minutes.
Sprinkle topping over pie; bake until pie is set and topping is golden 20 to 25 minutes. Cool completely before slicing.