Directions

1. Preheat oven to 400 degrees with rack in center. Line a rimmed baking sheet with parchment paper; set aside. In a large bowl, toss together raspberries, rhubarb, 1 1/4 cups sugar, tapioca, lemon juice, and a pinch of salt; stir to combine. Let stand 15 minutes, stirring once or twice. Divide filling among six 10-ounce baking dishes.
2. In a medium bowl, whisk together both flours, remaining 1/4 cup sugar, baking powder, baking soda, lemon zest, and remaining 1/2 teaspoon salt. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Add yogurt, and stir with a fork to form a moist dough with the consistency of thick cake batter. Using a large spoon, top filling with dollops of dough, leaving a 1-inch border.
3. Brush tops of dough with cream; sprinkle lightly with sanding sugar. Transfer baking dishes to prepared baking sheet; bake, rotating sheet halfway through, until biscuits are golden brown and juices are bubbling, 50 to 60 minutes. (If the biscuits are browning too quickly, cover loosely with aluminum foil during the last 15 minutes of baking.) Cool on a wire rack, at least 1 hour. Serve warm or at room temperature.