Stir together flour, sugar, and salt in a large bowl. Cut 1 cup butter into flour mixture with a pastry blender until mixture resembles a coarse meal.

Whisk together egg yolk and milk; stir into flour mixture just until dough starts to form a ball. (I used my food processor to pulse everything together and make the dough.)

Shape dough into a flat disk using lightly floured hands. Wrap disk in plastic wrap, and chill 1 to 24 hours.

Preheat oven to 425°.

Place peaches, plums, blueberries, and vanilla in a large bowl.

Stir together 1 3/4 cups sugar and 1/2 cup flour; sprinkle over peach mixture, and gently stir.

Spoon into a lightly greased 13 x 9-inch or shallow 3-qt. baking dish. Drizzle with melted butter.

Place dough disk on a lightly floured surface; sprinkle with flour.

Place a piece of plastic wrap over dough disk to make the dough easier to roll. Roll dough to 1/8-inch to 1/4-inch thickness; cut into 2-inch squares.

Arrange squares in a patchwork pattern over peach mixture, leaving openings for steam to escape.

Whisk together egg and 2 Tbsp. water; brush dough with egg mixture. Sprinkle with sanding sugar or sparkling sugar.

Bake on lowest oven rack at 425° for 40 to 55 minutes or until crust is deep golden and peach mixture is bubbly, shielding edges with foil during last 5 to 10 minutes to prevent excessive browning.

Transfer to a wire rack; cool 1 hour.