1. Preheat oven to 425 degrees, with rack in center; line bottom rack with foil. On a lightly floured work surface, roll out dough to a 13-inch round, about 1/8 inch thick. Transfer to a parchment-lined baking sheet, and chill 30 minutes.

2. Meanwhile, in a bowl, toss together grapes, sugar, lemon zest and juice, lavender, salt, and flour. Transfer to a sieve set over a bowl; drain 30 minutes.

3. Mound mixture in center of chilled dough, leaving a 2 1/2-inch border. Dot with butter. Fold edges of dough over filling, leaving center open. Brush dough with egg white; sprinkle with sugar.

4. Bake until pastry is golden brown, about 20 minutes. Reduce oven heat to 350 degrees, and loosely cover center with foil. Continue baking until juices are bubbling, 15 to 20 minutes more. Let cool.