Preheat the oven to 370 degrees F.
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, 30 to 35 minutes. Cool on a wire rack for at least 15 minutes before serving.
Yield: 8 to 10 servings