Mix in a bowl the flour, powdered sugar and salt. Cut in the butter until you get a crumb texture. Add 3 tbsp of water and mix until the dough comes together. Add an additional 1 tbsp at a time if need be. Wrap the dough in saran wrap and refrigerate for 2 hours.
Preheat oven to 350 F (175 C). Separate the dough into two. Roll each piece out on a floured surface into a 10 inch circle. Place the circles on a baking sheet lined with parchment paper.
Gently toss the blackberries with the sugar and cornstarch in a large bowl. Divide the berries evenly between the two circles of dough. Fold the edges over around the berries.
Lightly beat egg with milk, and brush edges with the egg. Sprinkle with the sugar. Bake for 35-45 minutes or until edges are golden brown and filling bubbly.
For the sauce, add 4 ounces of red wine and the sugar to a sauce pan. Let it boil until the sugar dissolves and then leave it to simmer for 3 minutes.
Mix the cornstarch and a little bit of the hot red wine sauce in a separate bowl until it is smooth.
Pour back into the pan and bring once again to a boil until thickened, stirring vigorously. Add a bit more wine if the sauce is to thick. Let the sauce cool a bit before pouring liberally over the galettes.
Blackberry Galette with Red Wine Sauce for a Red Wedding https://cultureatz.com/blackberry-galette-red-wine-sauce/