In the bowl of a food processor, fitted with chopping blade combine flour, 1/4 cup of the sugar, the butter, baking powder and salt; pulse until mixture resembles coarse crumbs. Set aside 1 tablespoon of the heavy cream; add the rest to the crumb mixture and process again until dough forms.
Turn the dough out onto a lightly floured surface and gently pat it together. Divide dough into 8 2-inch balls; flatten and let sit at least 20 minutes. Meanwhile, in a large bowl, combine the blueberries, nectarines, 3 tablespoons of the remaining sugar and the tapioca; let sit
Preheat oven to 350 F 175 C. Spread blueberry/nectarine mixture in a shallow, 2 1/2 quart baking dish; arrange dough rounds on top. Brush with remaining heavy cream and sprinkle with remaining 1 tablespoon sugar. Bake until the filling bubbles and the topping has browned, about 50 minutes.