Combine flour, sugar, and salt in the basin of a food processor³ and pulse to combine.
Scatter butter pieces over the top of your dry ingredients and pulse until butter is cut into the dry ingredients and the mixture resembles coarse crumbs.
Add sour cream and pulse again until the mixture begins to clump together. Transfer to a piece of plastic wrap or wax paper and form into a ball. Lightly flatten into a disc and chill for at least 30 minutes in the refrigerator.
While galette crust is chilling, preheat oven to 400F (205C) and prepare your fruit filling.
Combine fruit, sugar, lemon juice, cornstarch, and salt in a large bowl and stir until well-combined. Set aside.
Once dough has finished chilling, remove from refrigerator and transfer to a clean, lightly floured surface. Dust with additional flour and roll out to a 12-13” circle. Transfer to a baking sheet (sometimes I will use a round pizza pan, but a standard cookie sheet works, too).
Prepare egg wash by whisking together egg and water in a small bowl. Use a pastry brush to lightly brush your pie dough with egg wash (this will help keep the galette from getting soggy). Don’t discard egg wash yet.
Give your fruit one more stir in the bowl and then carefully transfer to the center of your galette crust, leaving a 2” perimeter of fruit-free crust.
Fold the perimeter/border of your crust carefully over the fruit filling and pinch together to secure (it will not completely cover it, you should have a nice circle of exposed fruit in the center!).
Dot the fruit with butter pieces, if using.
Brush the folded-over pastry with egg wash and sprinkle with sugar, if desired.
Transfer to 400F (205C) oven and bake for 30-35 minutes or until the fruit is bubbling and the crust is a nice golden color. Allow to cool before slicing and serving.
¹By prepared I mean rinsed, dried, stems/pits/skins removed as applicable, and sliced into 1/4" thick slices or bite-sized pieces. This was 3 heaping cups of fruit for me and I used 1 heaping cup each of raspberries, blueberries, and sliced peaches.²If using very juicy fruites (such as all peaches, juicy strawberries, etc.) then increase this to 2 Tablespoons of cornstarch so that your galette doesn’t become soggy.³If you don’t have a food processor, you can use a pastry cutter or grater to cut the butter into the dry ingredients and then stir in the sour cream.