This free-form dessert is simple but elegant. Cornmeal contributes texture and strength to the tender crust.
Preheat oven to 400°.
To prepare crust, lightly spoon 1 cup flour into a dry measuring cup; level with a knife. Combine 1 cup flour, cornmeal, 3 tablespoons granulated sugar, baking powder, and salt in a food processor; pulse 4 times or until blended. Add chilled butter; pulse 6 times or until mixture resembles coarse meal. With processor on, slowly pour orange juice through food chute, processing just until blended (do not allow dough to form a ball).
Press mixture gently into a 4-inch circle on plastic wrap; cover. Chill for 15 minutes. Slightly overlap 2 sheets of plastic wrap on a slightly damp surface. Unwrap and place chilled dough on plastic wrap. Cover dough with 2 additional sheets of overlapping plastic wrap. Roll dough, still covered, into an 11-inch circle. Remove top sheets of plastic wrap; place dough, plastic wrap side up, on a baking sheet lined with parchment paper. Remove plastic wrap. Sprinkle dough with 1 teaspoon flour.
To prepare filling, combine grapes, 2 tablespoons granulated sugar, cornstarch, vanilla, and cinnamon. Spoon into center of dough, leaving a 2-inch border. Fold edges of dough toward center, pressing gently to seal (dough will only partially cover grape mixture). Combine water and egg yolk; brush over edges of dough. Sprinkle turbinado sugar over grape mixture and dough, if desired. Bake at 400° for 25 minutes or until crust is brown. Serve warm or at room temperature.