Combine flour, sugar and salt in large mixing bowl. Work in the butter using a pastry cutter or your fingers until the mix resembles small peas. Add sour cream and cold water and use your hands to bring the dough together. Transfer dough to a sheet of parchment paper, flatten to a 5-inch round disk and wrap tightly. Refrigerate for 1 to 2 hours (or overnight), until firm.

Preheat oven to 400°F. In a medium bowl, combine fruit, salt, cinnamon, lemon juice, sugar and cornstarch. Toss gently to combine and set aside. Roll out dough on a floured surface into 1 (14-inch) round disk. Gently transfer dough to a parchment-lined baking sheet. Spoon fruit filling and juices into the center of the disk, leaving a 2- to 3-inch border for folding. Fold the border over fruit in pleats, leaving the center exposed.

Beat egg with water and brush egg wash over the crust. Sprinkle the top of the galette lightly with raw (or turbinado) sugar.

Bake 30 to 35 minutes, or until crust is golden and fruit is bubbly and thickened. Remove from oven and cool for 20 minutes on a wire rack. Cut into 6 even wedges. Serve warm or at room temperature topped with your favorite ice cream or gelato.