For the pastry:
- 1 1/2 cups all-purpose flour
- 1/2 tsp. salt
- 4 Tbs. (1/2 stick) chilled unsalted butter
- 1/4 cup vegetable shortening
- 3 to 4 Tbs. cold water
For the filling:
- 2 lb. plums, pitted and cut into 1/2-inch wedges
- 1 tsp. fresh lemon juice
- 1/4 cup plus 2 tsp. sugar
- 3 Tbs. all-purpose flour
- 1/2 tsp. ground cinnamon
- 1 Tbs. heavy cream
- 1/4 cup apricot jam, warmed and strained
Using floured hands, pat the dough into a smooth, flattened disk. Use immediately, or wrap in plastic wrap and refrigerate for up to 2 days.
Place a baking sheet upside down on the middle rack in an oven and preheat to 400°F.
On a lightly floured surface, roll out the dough to 1⁄4-inch thickness. Transfer to a 9-inch square tart pan, pressing the dough up the sides of the pan and leaving a 2-inch overhang.
To make the filling, in a large bowl, stir together the plums, lemon juice, the 1⁄4 cup sugar, the flour and cinnamon. Transfer to the tart shell and arrange the plums in an even layer. Fold the overhanging dough on top of the plums. Brush the dough with the cream and sprinkle with the 2 tsp. sugar.
Place the pan on the baking sheet in the oven. Bake until the crust is golden brown and the filling is bubbling, about 45 minutes. Transfer the pan to a wire rack and let cool for 1 hour. Brush the filling with the apricot jam.