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1
Line a 9-inch round tart pan or a rectangular pan measuring 4 x 13 inches wide with the dough. (If using the latter, you will have extra dough.) Chill while you preheat the oven to 375°F, then line with foil and pie weights and bake for 15 to 20 minutes. Remove the foil, then bake until golden, 12 to 15 minutes more.
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2
Lay the berries on a towel. Heat the jam with a few teaspoons of water to thin it, then press through a sieve. Brush about half on the tart shell. Starting with the larger berries, arrange them around the edge of the shell, working towards the center with progressively smaller fruits. Wedge them closer to one another, making a single layer. Return the tart to the oven for another 5 minutes.
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3
Reheat the remaining jam, then dab a little over each berry. Remove the tart from its pan, set on a serving plate, and dust the edges with confectioners' sugar.
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4
Put the flour, sugar, salt, and zest, if using, into a food processor; pulse to combine. Add the butter and pulse until the butter is broken up into pieces the size of baby peas. Drizzle in the water-vanilla mixture and pulse until large, moist-looking crumbs have formed.
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5
Gather the crumbs together into a small mass. If there is any dry flour left in the bowl, add a few more drops of water to bring it together as well, then add it to the rest of the dough. Shape the dough into a disk about an inch thick and refrigerate. If rolling the dough, refrigerate for about 30 minutes.
