1. Make the dough: Pulse flour, granulated sugar, and salt in a food processor. Add butter, and process until mixture resembles coarse meal, about 10 seconds. With the machine running, add ice water in a slow, steady stream until mixture just begins to hold together. Shape dough into 2 disks. Wrap each in plastic, and refrigerate 1 hour (or up to 2 days).

2. Make the jam: Combine grapes and lemon juice in a medium nonreactive saucepan over high heat. Cook, stirring frequently, until grapes release their juices, about 7 minutes. Strain through a fine sieve (you should have 1 1/2 to 2 cups juice). Return juice to saucepan over high heat, stir in granulated sugar and a pinch of salt, and bring to a boil. Reduce heat to medium, and simmer until temperature registers 220 degrees on a candy thermometer, about 8 minutes. Transfer jam to a bowl, and let cool, stirring occasionally.

3. On a lightly floured surface, roll out each disk of dough to 1/8-inch thickness. Transfer 1 round to a baking sheet, preferably lined with parchment, and fit the other round into a 9 1/2-inch tart pan with a removable bottom. Trim edges flush with tart pan. Freeze both until firm, about 15 minutes.

4. Using the wide base of 2 pastry tips (one 3/4 inch in diameter and one 1 inch), cut clusters of holes in dough on baking sheet to create a bunch of grapes. Using a paring knife, cut a stem at the top. Freeze until firm.

5. Preheat oven to 375 degrees. Spread 1 cup grape jam over dough in tart pan. (Refrigerate any remaining jam for up to 2 weeks.) Brush top edge of dough with egg. Slide remaining dough round on top, centering the design. Press edges to seal, and trim excess dough. Brush top with egg, then sprinkle with sanding sugar. Refrigerate for 30 minutes.

6. Bake tart for 15 minutes, then gently tap pan on counter to release air bubbles. Return to oven, and bake until golden brown and bubbling, 15 to 20 minutes. Transfer to a wire rack, and let cool. Unmold, and transfer to a platter. Serve with creme fraiche.