This galette is one of my favorite things to bake when I have an abundance of fresh berries. The strawberries and blueberries lend a sweet jam-like texture when baked, while the blackberries retain a firm bite for a contrast in textures. I continue to make this recipe even after berry season since it works beautifully with a mix of thawed frozen berries. I use a crust that mixes shortening and grated frozen butter for super flakiness, but you could also substitute your favorite homemade or store-bought pie dough.
For the crust: Whisk the flour, sugar and salt together in a medium bowl. Add the shortening and work it into the flour by smashing and smearing it in with your fingers until the shortening is
nearly incorporated. Use the large holes of a box grater to grate the frozen butter into the flour mixture. Use a whisk or plastic bowl scraper to toss the butter into the flour and break up the pieces until they are the size of peas. Combine the water and vinegar in a cup, then stir the mixture into the flour mixture. Press and fold the dough in the bowl a few times to incorporate any dry bits of flour.
Place two pieces of plastic wrap on a work surface so they overlap slightly and place the dough in the center. Fold one side of the plastic over and flatten the dough slightly. Repeat on all four sides to incorporate any last dry bits of dough, repeating if needed. Wrap the dough securely in the plastic, flatten it into a rough circle and chill at least 2 hours or overnight. (Before rolling, allow the dough to rest at room temperature for 5 minutes.)
For the filling: Combine the blueberries, strawberries, blackberries, sugar, cornstarch and salt in a medium bowl and toss until the berries are coated with the dry ingredients. Add the lemon zest, lemon juice and vanilla and toss to combine. Set aside.
Line a sheet pan with parchment paper. On a lightly floured surface, use a rolling pin to roll the dough into a circle about 17 inches wide. Use a knife or pizza cutter to trim the edges so they are even, creating a circle about 16 inches wide. Dust the top of the dough very lightly with flour and then roll the dough around the rolling pin and transfer it to the prepared sheet pan and unroll (it's okay if the dough hangs over the edge). Patch any cracks with scraps of dough moistened with water. This will prevent your galette from leaking as it bakes.
Pour the berry filling and any accumulated juices into the center of the dough. Spread the filling out, leaving a 3-inch border around the edges. Fold the border of dough over the fruit, allowing the dough to pleat as you fold. Press the dough gently down at the pleats to ensure that it seals. Brush the dough with the cream and sprinkle with the sugar. Place the galette in the freezer for 20 minutes.
While the galette chills, preheat the oven to 425 degrees F. Bake the galette for 10 minutes, then reduce the temperature to 400 degrees F and bake until the crust is golden and the fruit is bubbling, 45 to 50 minutes longer; rotate the pan during the last 20 minutes of baking if the galette is browning unevenly. Cool at least 2 hours before slicing and serving.