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.
This is a fine basic recipe. I use 2 TB bourbon and 1 TB ice water in the crust for greater tenderness. I've also made this with 4 cups of sliced fresh peaches and 1 TB of melted low-sugar apricot jam on top. I find 1 TB of jam to be plenty, and the extra sugar to be unnecessary. Crust is very nice, it's easy to make, and with a bit of yogurt or ice cream, pretty delicious.
This was delicious, and the plum jam is great on its own! Super easy. I used a store-bought pie crust and saved myself a lot of time. One suggestion I would make is not to spread the tablespoon of jam on the crust until about the last 10 minutes. 20 minutes was too early, as it burned.