PREP: Mise the recipe ahead of time.

1. Mix the sour cream pastry dough:

• Measure out the flour onto a sanitized, thoroughly dry work surface. Add the sugar and salt, and then use a fork to thoroughly mix the dry ingredients.
• Place the step 1 butter onto the dry ingredients and use a bench scraper to chop the butter into pieces a little larger than the size of a pencil eraser. Work quickly so that the ingredients remain cold.
• Scrape the flour mixture into a bowl.
• Add the sour cream and about 2 Tbsp ice-cold water into the bowl and mix with the fork. Mix in just a little more iced water to make a shaggy yet cohesive dough. Use your hand to continue mixing until the dough forms a ball. Don’t overwork the dough or make it too wet.
• Dust a little flour on the work surface and place the dough ball on it. Use your hands to press and form the dough into a round disk about ¾-inch thick.
• Wrap the dough disk in plastic wrap and refrigerate at least 30 minutes or up to 24 hours.

2. Mix the sour cream filling:

• Crack the egg into a bowl and beat it with a fork to combine the yolk and white.
• Beat in the sugar, then beat in the flour until smooth.
• Beat in the sour cream and vanilla.
• If prepping ahead, scrape the filling into a container and refrigerate.

HOLD: Refrigerate the dough up to 24 hours or freeze up to 1 month; refrigerate the filling up to 3 days.

BAKE! Assemble and bake the galette.

3. Preheat an oven to 400°F. Line a half-sheet tray with parchment.

4. Roll out and rest the dough:

• Dust a sanitized, thoroughly dry work surface with a little flour and rub some flour onto a rolling pin.
• Place the dough disk on the floured surface and roll out into a 14-inch circle.
• Transfer the dough circle to the sheet tray. (The dough will extend an inch or so beyond the edges of the tray on two sides.)
• Place the tray in the refrigerator.

5. Fabricate and sweeten the fruit:

• Wash the cherries and blot dry on paper towels.
• Remove the stems from the cherries. Use a paring knife to cut each cherry into quarters around the pit, and then gently pull the cherry flesh off of the pits. Put the cherry quarters into a bowl.
• Inspect the blueberries and remove any stems. Wash the blueberries, blot dry on paper towels, and add them to the bowl.
• Inspect the blackberries and remove any leaves or twigs.  Don’t wash them.
• Sprinkle the step 5 sugar over the berries and gently fold it in to evenly coat the fruit.

6. Assemble the galette:

• Melt the step 6 butter in a microwave oven or on the stovetop.
• Scrape the sour cream filling onto the center of the dough circle and spread it out, leaving a 2-inch rim of un-filled dough around the edges.
• Mound the fruit onto the filling.
• Fold up the dough edges around the fruit filling, making large pleats as you work your way around. Press firmly on the rim of the dough circle to prevent the crust from opening out as it bakes.
• Sprinkle the dough rim with the step 6 granulated sugar.
• Drizzle the melted butter onto the fruit.
• Sprinkle the raw sugar over the fruit.

7. Bake the galette:

• Immediately place the galette in the oven and set a timer for 10 minutes.
• In ten minutes check the galette and, if the crust rim is beginning to open out, use a flexible spatula to press it back down into shape.
• Continue baking for about 30 minutes longer until the fruit is bubbling and the crust acquires a rich, golden brown color.

8. Cool the galette:

• Remove the galette from the oven to a cool place.
• Slide a spatula or other kitchen tool under the galette to slightly raise the bottom crust off the tray, allowing air to circulate underneath.
• Cool to room temperature. (If you try to slice the galette while hot or warm, the filing will ooze out.)

9. Serve:

• Transfer the galette to a cutting board and slice into 8 wedges.