Late-Summer Mixed-Fruit Pie

Ingredients

• 3 medium plums, apricots, or nectarines
• 9 ounces pitted cherries , from about 12 ounces whole fruit
• 6 ounces fresh blueberries, blackberries, or raspberries
• 4 ounces Cape gooseberries, currants, or fresh cranberries
• 7 ounces sugar
• 1/2 teaspoon Diamond Crystal kosher salt; for table salt, use half as much by volume or use the same weight
• 1 1/2 ounces tapioca starch , such as Bob's Red Mill
• Old-Fashioned Flaky Pie Dough, rolled and chilled as per the directions for a double crust
• For the Egg Wash :
• 1 large egg
• 1 large egg yolk
• 1/2 ounce heavy cream
• 1/8 teaspoon Diamond Crystal kosher salt; for table salt, use half as much by volume or use the same weight

Directions

1. For the filling, rinse the plums, leaving the skins on, then slice them 1/2-inch thick. Put 9 ounces (1 1/2 cups; 255g) into a large bowl. Add the pitted cherries, blueberries, and Cape gooseberries, or the fruit combination you have chosen, until the fruit weighs 28 ounces (790g; volume will vary). Sprinkle in the sugar, salt, and tapioca starch, then use a flexible spatula to fold everything together thoroughly. Transfer the fruit mixture to the prepared pie shell, then cover it with the remaining dough in whichever pattern you like: a full top crust, lattice, herringbone, or checkerboard. Cut off the extra dough, then chill the pie so the top crust is cold all the way through, about 30 minutes. While it chills, set an oven rack in the lower-middle spot and heat the oven to 400 degrees F.
2. For the egg wash, if you are using it, whisk the egg, egg yolk, cream, and salt together in a small bowl. Paint a thin, even coat over the cold top crust. It will make the pastry shiny and golden, but the pie does not depend on it.
3. Set the chilled pie on a rimmed baking sheet lined with parchment. Bake until the crust has turned golden, about 1 hour, then drape it loosely with tented foil. As another option, put an empty baking sheet on the highest oven rack to act as a heat shield. Bake longer, until the fruit is bubbling at the center as well as the edges, about 15 minutes more. If the top crust fully encloses the fruit, use a digital thermometer and bake until the pie registers 212 degrees F inside.
4. To serve, cool the pie until a digital thermometer inserted in the center reads 85 degrees F or lower, about 3 hours, with the exact timing depending on the pie plate. If it is warmer than that, the filling will be thin and loose. Cut wedges with a sharp knife, pressing down firmly along the bottom and sides of the pie plate so the bottom crust is fully sliced. Cover leftovers in foil and store at room temperature for up to 3 days; before serving, refresh the crust for 10 minutes in a 350 degrees F oven.