Make the dough:
Combine the flour, sugar, lemon zest, and salt in a food processor and pulse briefly to combine. Add the butter pieces and pulse until the mixture resembles coarse crumbs. Drizzle the water evenly over the crumbs and process just until the dough is moist but still crumbly. Turn the dough out onto the counter and shape it into a 5-inch disk. Wrap and refrigerate until firm and chilled, at least 1 hour.
Fill and bake the galette
Heat the oven to 400°F. In a large bowl, whisk the sugar, flour, and lemon zest. Add the plums and vanilla and toss until combined. Set aside.
Roll the dough between two large pieces of parchment, lightly flouring when necessary, to form a rough round slightly larger than 14 inches and about 1/8 inch thick. Trim off excess dough to make an even 14-inch round. Roll the dough around the rolling pin and transfer it to a parchment-lined rimmed baking sheet or jelly roll pan. The dough will hang slightly over the edges of the pan for now.
Pile the filling into the center of the dough, leaving a 3-inch rim of dough. Fold the dough edge up and over the filling, pleating the dough as you go. (The galette will flatten out as it bakes.) Press the pleats gently to seal. Brush the dough with the milk and sprinkle the dough and fruit with the crushed sugar cubes, pressing on the sugar lightly to make it adhere. Bake until the fruit is tender and the crust is browned, 45 to 50 minutes. Serve warm or at room temperature.
Make Ahead Tips
You can make and chill the dough for this rustic tart a day in advance. You can also bake the galette several hours ahead and keep it at room temperature.
nutrition information (per serving):
based on ten servings;
Photo: Mark Ferri