Though a beloved pie classic, rhubarb has a strong tartness that sometimes needs moderating. Sweet raspberries do that job perfectly here, while a splash of crème de cassis adds even deeper berry flavor. A slice of this pie absolutely screams for a scoop of vanilla ice cream.
1. Preheat oven to 350°.
2. Place tapioca in a spice or coffee grinder; process until finely ground. Combine tapioca, raspberries, and next 5 ingredients (through 1/8 teaspoon salt) in a bowl; toss well. Let raspberry mixture stand 10 minutes; stir to combine.
3. Roll 1 (9-inch) dough portion into an 11-inch circle. Fit dough into a 9-inch pie plate coated with cooking spray, draping excess dough over edges. Spoon raspberry mixture and any remaining liquid into dough. Fold edges under; flute. Bake at 350° for 40 minutes.
4. While pie bakes, place flour and remaining ingredients in a food processor; pulse 10 times or until mixture resembles coarse crumbs.
5. Increase the oven temperature to 375°.
6. Sprinkle topping evenly over pie. Bake at 375° for 15 minutes or until topping is golden brown and filling is thick and bubbly. Cool completely on a wire rack.