Plum Blackberry Galette

Recipe courtesy of Martha Stewart.

Ingredients:
For Dough:
2 sticks cold unsalted butter, cut into small pieces
2 1/2 cups all-purpose flour
1 teaspoon coarse salt
1 teaspoon sugar
1/2 cup ice water

For Filling:
2 tablespoons all-purpose flour, plus more for parchment
2 pounds mixed plums (about 8), halved, pitted, and cut into 1/2-inch wedges if large
1 teaspoon fresh lemon juice
Pinch of coarse salt
3/4 cup plus 1 tablespoon granulated sugar, divided
6 ounces blackberries
1 tablespoon unsalted butter, cut into small pieces
1 large egg, lightly beaten
2 tablespoons coarse sanding sugar

Directions:
For Dough:
Lay roughly three quarters of butter on a parchment-lined baking sheet and freeze until hard, at least 30 minutes. Meanwhile, refrigerate remaining butter.

Combine flour, salt, and sugar in a food processor. Add refrigerated butter and pulse to combine, about 10 times. Add frozen butter and pulse until mixture resembles coarse meal with some blueberry-size clumps.

Add ice water and immediately pulse until just incorporated, about 10 times more. Squeeze a small amount of dough to make sure it holds together. Pulse a few more times, if necessary.

Empty dough onto a piece of plastic wrap. Bring edges together to gather dough, pressing so it comes together into a mass. Form dough into a disk and wrap in plastic. Roll out dough, still wrapped in plastic, to a 1/2-inch-thick disk. Refrigerate at least 45 minutes and up to 2 days; dough can be wrapped in plastic, then in foil, and frozen up to 1 month.

Roll out dough to an 18-inch round on lightly floured parchment (parchment should measure at least 18 inches square; overlap 2 pieces, if necessary).

For Filling:
Combine plums, lemon juice, flour, salt, and 3/4 cup granulated sugar. Gently stir in blackberries and place in middle of pastry, leaving a 3-inch border. Dot top of fruit mixture with butter. Fold up edges of pastry to partially cover fruit; brush pastry with egg wash and sprinkle with sanding sugar. Slide galette on parchment onto a rimmed baking sheet (trim parchment to fit, if necessary). Refrigerate until dough is firm, about 30 minutes.

Meanwhile, preheat oven to 425 degrees. Bake galette 30 minutes. Reduce heat to 375 degrees and bake until pastry is golden and juices are bubbling, about 30 minutes more. Let cool completely on a wire rack, about 2 hours.