Put the flour, butter, and salt in a food processor and process for 5 seconds; the butter should still be in pieces. Add the ice water and process for 5 seconds longer, just until the dough comes together; the butter should still be visible.

Remove the dough from the processor and gather it into a ball. If your kitchen is warm, chill the dough by wrapping it and refrigerating it for 30 minutes. On a lightly floured surface or sheet of parchment, roll out the dough into a 16-by-18-inch oval 1/16- to ⅛-inch thick. Drape the dough over the rolling pin and transfer it to a large, heavy baking sheet. Chill the dough until firm, about 20 minutes. Preheat the oven to 400°F. Make the Filling

In a small bowl, combine ¼ cup of the sugar with the ground almonds and flour. Spread this mixture evenly over the dough to within 2 inches of the edge. Arrange the plum wedges on top and dot with the butter. Sprinkle all but 1 teaspoon of the remaining ⅓ cup sugar over the fruit. Fold the edge of the dough up over the plums to create a 2-inch border. (If the dough feels cold and firm, wait for a few minutes until it softens to prevent it from cracking.) Sprinkle the border with the reserved 1 teaspoon of sugar.

Bake the galette in the middle of the oven for about 1 hour, until the fruit is very soft and the crust is richly browned. If any juices have leaked onto the baking sheet, slide a knife under the galette to release it from the sheet. Evenly brush the preserves over the hot fruit; brush some up onto the crust, too, if desired. Let the galette cool to room temperature before serving. Top with ice cream if desired.