Roll out the dough on a lightly floured work surface into an 11-inch round about 1/8-inch thick. Transfer the dough to a 9-inch pie pan. Trim off any overhanging dough to make a 1/2-inch wide overhang around the edge of the pan. Flute the edges of the dough with your fingertips.
In a large bowl, mix the rhubarb, apples, sugar, 1/2 cup of the brown sugar, the cornstarch, lemon juice, and brandy, and toss to coat the fruit evenly. Pour the mixture into the piecrust.
In a medium bowl, combine the flour, walnuts, remaining 1/2 cup brown sugar, butter, cinnamon and nutmeg and mix well with your fingertips until the mixture resembles coarse crumbs. Sprinkle evenly over the filling.
Bake until the top is golden brown and the juices are bubbling, 40 to 45 minutes. Cool on a wire rack for at least 15 minutes before serving.
Basic Sweet Piecrust:
In a mixing bowl, combine the flour, sugar, and salt. Mix well. Add the shortening and mix until the mixture resembles coarse crumbs. Add the water and let sit 1 minute. Using either a fork or your hands, carefully press the mixture together to form a soft ball. Wrap in plastic wrap and refrigerate at least 30 minutes.
Preheat oven to 350 degrees F.
Remove the dough from the refrigerator and place it on a lightly floured surface. Roll out the dough on the floured surface into a circle about 12-inches in diameter and 1/8-inch thick. Gently fold the circle of dough in half and then in half again so that you can lift it without tearing it, and unfold into a 9-inch by 2-inch deep-dish pie pan. Crimp the edges and place in the oven.
Bake until golden, about 15 to 20 minutes. Remove from the oven and cool completely.