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 15min and then reduce to 350F for another 15-25min, 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.