INGREDIENTS
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1 lemon
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1 cup all-purpose flour
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1 cup light brown sugar
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1 1/2 teaspoons ground cinnamon
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1/2 teaspoon kosher salt, plus more
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1/2 cup (1 stick) unsalted butter, melted, cooled
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3/4 cup pistachios or sliced almonds
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12 ounces blackberries (from 2 6-ounce containers), halved crosswise if large
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1 tablespoon cornstarch