Make the pastry: Combine the flour, sugar, and salt in a large bowl. Sprinkle butter over dough and using a pastry blender or your fingertips, work it into the flour until the mixture resembles small peas. Sprinkle sour cream and 3 tablespoons water over mixture and stir/mash it together to combine; it should form large clumps; add last tablespoon water if it does not. Use your hands to bring it together into a single mass. Transfer dough to a large square of parchment paper, patting it into a flatter packet, and wrap it tightly. Chill in the fridge until firm, 1 to 2 hours or up to 4 days.

Make filling: Combine fruit, salt, citrus juice (if using), sugar, and starch in a medium bowl and set aside.

Assemble galette: Heat oven to 400°F and flatten the parchment paper that you wrapped your dough in on a large baking sheet. On a floured counter, roll the dough out into a large round-ish shape, about 14 inches across. Gently transfer it to the parchment paper in the pan. Spoon fruit filling and any juices that have collected into center, leaving a 2- to 3-inch border uncovered. Fold this border over fruit, pleating the edge to make it fit. The center will be open.

For a darker, glossier crust, beat egg with 1 teaspoon of water and brush it over the crust. Sprinkle it all over with turbinado or coarse sugar.

Bake galette: For 30 to 35 minutes, or until golden all over and the fruit is bubbling and juicy. Cool for at least 20 minutes on wire rack. Serve warm or at room temperature.