1. Preheat oven to 400°.
2. To prepare the crust, lightly spoon flour into dry measuring cups; level with a knife. Combine flour, cornmeal, granulated sugar, baking powder, and salt in a food processor; pulse 4 times or until blended. Add butter; pulse 10 seconds or until mixture resembles coarse meal. With processor on, slowly pour ice water through the food chute, processing just until blended (do not allow the dough to form a ball).
3. 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 barely damp surface. Unwrap chilled dough, and place on plastic wrap. Cover dough with 2 additional sheets of overlapping plastic wrap. With a rolling pin, roll the dough 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.
4. To prepare filling, combine apples, sugar, cinnamon, and salt. Arrange in center of dough, leaving a 2-inch border. Fold edges of dough toward the center, pressing gently to seal (dough will only partially cover apple mixture). Evenly sprinkle 1 teaspoon chopped butter over exposed fruit. Combine water and egg yolk; brush over edges of dough. Sprinkle turbinado sugar over apple mixture and dough, if desired. Bake at 400° for 25 minutes or until the crust is brown. Serve warm or at room temperature.