Make Dough: Start by making the pastry dough. Add the flour and salt to a large bowl and slightly whisk. Using two knives or a pastry cutter, cut in the butter until it resembles course sand. In a small bowl, whisk together the sour cream, lemon juice and water and add this to the butter-flour mixture. Using a wooden spoon or your fingers, mix in the liquid until large lumps form. Pat the lumps into a ball; do not overwork the dough. Cover with plastic wrap and refrigerate for 1 hour.
Make Filling: In a bowl toss the peaches, blueberries and sugar together.
Preheat Oven: Preheat oven to 400 F degrees. Line a baking sheet with parchment paper.
Roll the Dough: On a floured work surface, roll the dough out into a 12-14 inch round. Transfer to the prepared baking sheet.
Assemble: Arrange the peaches over the dough, leaving a 2-inch border. Fold the border over the filling, pleating the edge to make it fit. The center will be open. Brush crust with egg yolk. Sprinkle with more sugar if preferred and sprinkle some almonds over the crust.
Bake: Bake for about 45 minutes or until golden brown.
Nutritional information is for 1/6 of galette.
Keep your galette at room temperature on the day you have baked it and wrap any leftovers in plastic or an airtight container and store at room temperature.