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Line a baking sheet with parchment paper; set aside. On a lightly floured work surface, roll out puff pastry dough to a 17 1/2-by-9 1/2-inch rectangle, about 1/8 inch thick. Using a dry pastry brush, sweep off excess flour. Using a pastry cutter or pizza wheel, trim to a 16-by-8-inch rectangle. Cut out eight 4-inch squares. Place squares on prepared baking sheet; cover with plastic wrap and chill until firm, at least 1 hour or up to 8 hours.
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In a small bowl, mix together sugar and vanilla bean seeds until well combined. Transfer to a medium bowl along with rhubarb, salt, cornstarch, lemon juice and zest; mix until well combined. Gently fold in raspberries. Let filling stand until juicy.
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In a small bowl, whisk together egg yolk and cream; set egg wash aside. Stir filling and strain, discarding excess liquid, and divide equally among the pastry squares, placing it in the center. Brush edges with egg wash. Fold dough diagonally over filling to form a triangle, and press to seal. If desired, crimp the edges using the floured tines of a fork. Place filled turnovers on a baking sheet, covered with plastic wrap, and freeze until firm, 20 to 30 minutes. The filled turnovers can be frozen for up to 3 weeks; no need to thaw before proceeding.
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Preheat oven to 375 degrees. Brush tops of turnovers with egg wash, being careful not to let it drip over the edges, as this will prevent even puffing. Sprinkle generously with sanding sugar, and bake, rotating sheet halfway through, until turnovers are puffed and deep golden, 25 to 35 minutes. Using a thin metal spatula, immediately transfer turnovers to a wire rack to cool. They are best eaten the same day they are baked.
