To make lavender sugar, pulse 2 teaspoons chopped fresh lavender buds with 1/2 cup sugar in a food processor.
Preheat oven to 400 degrees. On a lightly floured surface, roll out pastry 1/8 inch thick. Cut into 8 squares, about 5 inches each; arrange on 2 parchment-lined baking sheets.
Stir together rhubarb, strawberries, lavender sugar, cornstarch, and salt. Arrange 1/3 cup mixture in center of each pastry square. Fold edges in; crimp to form rounds. Brush edges with beaten egg white and sprinkle with sanding sugar. Freeze until firm, 20 minutes.
Bake, rotating sheet halfway through, until golden brown and fruit is bubbling, 30 to 35 minutes. Transfer to a wire rack to cool. Serve with whipped cream.