Roll pastry for a single-crust pie to a 10-inch circle; place on a parchment-lined rimmed baking sheet.
Drop 1/4 cup mascarpone in berry-sized pieces over the bottom of the crust.
Stir 1 tbsp flour into 1/2 cup sugar, and toss with 2 chopped peaches, 2 chopped plums and 1/2 cup blueberries in a large bowl.
Pile onto the crust, leaving an inch around the edge.
Fold the edge over all around to enclose the fruit, and bake at 400°F for 30-40 minutes, until golden.
Whip 1 cup heavy cream with 1/2 cup mascarpone, 1 tbsp sugar and 1/2 tsp vanilla to dollop on top.