To make the pie crust (Directions for already made pie crust below in note).
Preheat oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper.
Put the flour, salt and sugar into the food processor. Pulse about 3-4 times. Then add the ½ inch cubed cold unsalted butter into the food processor with the flour mixture and toss until the butter is coated with the flour.
Cut the flour into the butter with the food processor until the mixture is a bit smaller than pea sized. If you see pieces that are bigger than pea sized that is okay.
Add the water and pulse until the dough starts sticking and coming together and the dough mixture resembles coarse meal. Dump the dough onto a lightly floured surface and roll dough into a ball. Divide the dough into 6 equal part and roll into 6 small balls. Chill for 30 minutes.
Roll out each ball of dough into a 6 inch circle. Place the dough with parchment paper on a baking sheet. Evenly distribute the sweet potato mixture between the six discs (a bit less than ¼ cup per disc). Gently fold the edges of the disc towards the sweet potato mixture *(see photos above). Press the folded and gathered areas gently to to adhere the folds.
Make an egg wash by beating together the egg yolk and milk. Using a pastry brush, brush edges of each galette with the egg wash and sprinkle the crust with turbinado or granulated sugar (optional).
Bake for 25-30 minutes or until golden. Serve warm or at room temperature with whipped cream if you like. Store galettes in the fridge for up to 3-4 days. See *notes, if you want use store bought pie crust.