- 1/2 pound fresh rhubarb, cut crosswise into 1/2-inch-thick slices (about 2 cups)
- 1/2 cup sugar
- 1 tablespoon water
- 1/2 cup raspberry jam
- 2 cups all purpose flour
- 2 tablespoons sugar
- 2 teaspoons baking powder
- 1/2 teaspoon salt
- 10 tablespoons (1 1/4 sticks) frozen unsalted butter
- 2/3 cup (or more) ice water
- 1 tablespoon whipping cream beaten with 1 tablespoon sugar (for glaze)
- Vanilla Custard Sauce
- Combine rhubarb, sugar, and 1 tablespoon water in medium saucepan. Stir over medium heat until sugar dissolves. Cover; cook until rhubarb softens to thick pulp, stirring often, about 8 minutes. Mix in jam; cool. do ahead Can be made 1 week ahead. Cover; chill.
- Sift flour, sugar, baking powder, and salt into large bowl. Using large holes on box grater, grate frozen butter into bowl; stir to blend. Cover and freeze at least 15 minutes. DO AHEAD: Can be made 1 day ahead. Keep frozen.
- Preheat oven to 400°F. Line large rimmed baking sheet with parchment paper. Add 2/3 cup ice water to flour mixture, tossing until soft dough forms, adding more ice water by tablespoonfuls if dough is stiff. Transfer to lightly floured surface; knead gently 5 or 6 turns (butter should remain in firm pieces). Roll out dough to 12×9-inch rectangle. Spread 6 tablespoons filling over dough, leaving 1-inch plain border on all sides. Starting at 1 long side, roll up jelly-roll style; seal seam. Place seam side down on prepared sheet. Seal ends. Brush glaze over pastry.
- Bake pastry until golden (jam may leak out), about 30 minutes. Cool pastry briefly.
- Cut roly-poly crosswise into 6 to 8 rounds. Serve warm with Vanilla Custard Sauce and remaining filling.