Make the crust: In a food processor, pulse flour, cornmeal, sugar, and salt several times to combine. Add butter and pulse until mixture resembles coarse meal, with a few pea-size pieces remaining. Add 2 tablespoons ice water; pulse until dough is crumbly but holds together when squeezed. (If needed, add up to 2 tablespoons more ice water, 1 tablespoon at a time.) Do not overmix.

Turn dough out onto a floured work surface; knead once or twice. Flatten dough into a disk; wrap it in plastic, and refrigerate at least 1 hour.

Line a rimmed baking sheet with aluminum foil. Flour a large (about 16 inches long) piece of parchment paper. Place dough on paper. Using your knuckles, press edges of dough so it doesnt crack during rolling. Lightly flour top of dough to prevent sticking; roll out to a 14-inch round. Transfer dough (still on parchment) to prepared baking sheet.

Make the filling and bake: Preheat oven to 400 degrees. In a large bowl, toss together plums, sugar, and flour. Mound plum mixture in center of prepared crust, leaving a 2-inch border all around. Fold border over fruit in a pleated pattern. Brush dough with egg wash.

Bake tart until crust is brown and filling is bubbling (see note), about 45 minutes. Transfer baking sheet to a rack; let cool 20 minutes. Slice and serve warm or at room temperature.