1. Preheat oven to 425 degrees.with rack in lower third. On a lightly floured work surface, roll out pate brisee to 1/8 inch thick. Trim edges to form an 18-by-16-inch rectangle, and transfer to a parchment-lined baking sheet. In a small bowl, combine flour, hazelnuts, brown sugar, cornstarch, and salt; spread to cover middle of dough, leaving a 3-inch border all around.

2. Arrange plums in rows on top, slightly overlapping the slices and alternating direction of each row. Sprinkle plums with granulated sugar. Fold dough to enclose edges; brush dough with egg wash. Chill 30 minutes in refrigerator.

3. Bake 10 minutes. Reduce oven heat to 400 degrees. Bake until pastry is golden brown and plums are softened, about 30 minutes. Transfer to a wire rack; let cool to room temperature. Heat jam in a small saucepan over low heat, stirring until melted. Let cool a few minutes; brush evenly over plum slices. Serve.