Make the galette crust: In a medium/large mixing bowl, whisk together the flour, salt, and sugar. Cut the cold butter into chunks and add to the bowl. Cut in the butter using a pastry cutter, until the pieces of butter are pea-sized. Add the cold water and the sour cream, and start gently mixing it into the dough with a fork. Once the mixture starts to resemble a dough, use your hands to gently work it into a ball. Flatten the ball slightly into a thick disc of dough, wrap tightly in plastic wrap, and chill in the freezer for 15 minutes.
Make the filling: While the dough chills, mix up the filling. If not using blueberries and cherries, see recipe note below. Halve and pit your cherries using a cherry pitter or slice them all the way around, twisting the halves apart and removing the cherry pit with a knife. Add the blueberries, halved cherries, sugar, flour, lemon juice, and almond extract to a medium mixing bowl and stir gently with a spatula to combine everything. Set aside.
Roll the crust: Preheat the oven to 400 degrees. Get out a pizza pan or a large cookie sheet and line with parchment paper. Flour the counter and your rolling pin and remove the dough from the freezer. Take off the plastic wrap. Begin rolling out the dough, gently, turning continuously so it doesn't stick to the counter. Be patient and gently here - we're not trying to stretch the dough all the way out in a few passes of the rolling pin! That leads to tougher dough, and cracking of the dough. Once the dough is about 12-14 inches in diameter, gently roll it onto the rolling pin and transfer it onto the prepared baking sheet. (See note) Assemble and bake: Pour your fruit filling into the center of the dough and spread into an even-ish layer. Fold the edges of the crust up over the filling, working in small sections. This is a rustic dessert - it doesn't have to look perfect! Scraggly edges are encouraged here. Beat an egg in a small dish and brush it over the pie crust - this is your egg wash. Sprinkle the whole thing with coarse sugar. Bake for 28-35 minutes, or until crust is golden brown and sounds solid/crisp when tapped (it shouldn't really be soft). Filling will be a little bubbly too.
Serve and store: Slice and serve the galette immediately, with vanilla ice cream. Store leftovers, covered tightly, at room temp for up to 2 days. The crust will lose some of it's crisp-ness after this so it's best to eat it sooner!