1/4 cup sugar
1/4 teaspoon ground cinnamon
3 small ripe but firm plums, about 10 ounces, pitted and thinly sliced.
1 large egg, lightly beaten
2 teaspoons apricot jam, well-stirred
Preheat oven to 440ºF.
Line a baking sheet with parchment paper. Unfold thawed puff pastry sheet on top of paper. Prick all over with a fork so you won’t get large air bubbles in the crust as tart bakes. Place in freezer for 15 minutes.
Combine sugar and cinnamon in a small bowl. Sprinkle chilled puff pastry with 2 tablespoons cinnamon sugar. Arrange plums, overlapping them slightly, on top of the pastry, leaving 1 1/2″ border of pastry all around. Sprinkle with remaining 2 tablespoons cinnamon sugar. Brush edges of pastry with egg.
Bake until crust is golden brown, 25 to 30 minutes. Brush plums with apricot jam as soon as tart comes out of oven. Let cool completely on baking sheet, slice, and serve.
Makes 4 to 6 servings.