Cornmeal contributes nutty flavor and wonderful texture to the crust for this freeform summer-fruit tart.
Pulse flour, cornmeal, granulated sugar, and salt in a food processor until combined. Add butter; pulse until mixture resembles coarse meal. Drizzle in 6 tablespoons ice water evenly. Pulse until dough holds together when pressed between 2 fingers (it should not be wet or sticky). If too dry, add more water, 1 tablespoon at a time; continue pulsing. Shape into a disk; wrap in plastic. Refrigerate until firm, about 1 hour; or freeze up to 3 months.
In a large bowl, combine apricots, cherries, granulated sugar, cornstarch, and salt. On a lightly floured piece of parchment, roll out dough to a 15-inch circle. Transfer to a 10-inch pie dish; pour filling into crust. Fold edges of crust over filling, overlapping as necessary. Brush edges of dough with egg wash and sprinkle with sanding sugar. Refrigerate until firm, about 30 minutes.
Preheat oven to 375 degrees, with racks in middle and bottom. Place a foil-lined baking sheet on bottom rack. Bake on middle rack until pastry is golden and juices are bubbling in center, about 1 hour, 15 minutes. (If crust is browning too quickly, tent with foil.) Transfer to a wire rack; let cool at least 2 hours before serving. Pie can be stored at room temperature up to 1 day.