Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, butter, and salt in a food processor; process until mixture resembles coarse meal. With processor on, slowly add ice water through food chute, processing just until combined (do not form a ball). Press mixture gently into a 4-inch circle on plastic wrap. Cover and chill 30 minutes.
Preheat oven to 425°.
Line a baking sheet with parchment paper; sprinkle paper with cornmeal. Unwrap dough; roll dough into a 9-inch circle on a lightly floured surface. Place dough on baking sheet. Combine plums and cornstarch in a large bowl, tossing to coat. Add 1/4 cup Plum Jam; toss well to coat. Arrange plum mixture on top of dough, leaving a 1 1/2-inch border. Fold edges of dough over plum mixture. Bake at 425º for 20 minutes.
Remove galette from oven (do not turn oven off). Brush crust with 1 tablespoon Plum Jam; sprinkle galette with sugar. Bake at 425º for 20 minutes or until crust is golden brown. Cool in pan on a wire rack 10 minutes before serving.