1. Preheat oven to 425°.
2. Place pie dough on a lightly floured work surface; roll into a 12-inch circle. Fit dough into a 9-inch pie plate coated with cooking spray. Turn edges under; flute. Combine rhubarb, granulated sugar, juice, and apples; toss. Sprinkle the rhubarb mixture with cinnamon, 1/4 teaspoon salt, and 3 tablespoons flour; toss. Spoon rhubarb mixture into prepared crust.
3. Weigh or lightly spoon remaining 3.38 ounces flour (about 3/4 cup) into a dry measuring cup; level with a knife. Combine 3.38 ounces flour, remaining 1/8 teaspoon salt, and brown sugar in a medium bowl; cut butter into flour mixture with a pastry blender or two knives until mixture resembles coarse meal. Stir in walnuts. Sprinkle butter mixture evenly over rhubarb mixture. Bake at 425° for 15 minutes.
4. Reduce oven temperature to 375° (do not remove pie). Bake at 375° for 30 minutes or until golden and bubbly (shield edges of crust with foil if it gets too brown). Let pie stand on a cooling rack for 15 minutes before slicing.