1. Divide dough evenly into 8 pieces. On a lightly floured surface, roll out each piece to a 7-inch round, 1/8 inch thick. Transfer rounds to 2 large parchment-lined rimmed baking sheets, arranging several inches apart. (If rounds become too soft to handle, refrigerate until firm, about 20 minutes.)

2. In a large bowl, toss to combine rhubarb, raspberries, cornstarch, and granulated sugar.

3. Cover each round of dough with a heaping 1/2 cup rhubarb mixture, leaving a 1-inch border. Fold edges over rhubarb filling, leaving an opening in center; gently brush water between folds, and press gently so that folds adhere. Refrigerate or freeze until firm, about 30 minutes.

4. Preheat oven to 400 degrees. Brush edges of dough with water, and sprinkle with sanding sugar. Bake until crusts are golden brown, about 30 minutes. Reduce heat to 375 degrees, and bake until juices bubble and start to run out from center of each galette, 15 minutes more. Transfer to a wire rack, and let cool completely before serving.