Preheat oven to 350°F. On a lightly floured piece of parchment, roll out dough into an approximate 16-inch oval, 1/4 inch thick. Transfer dough (on parchment) to a large baking sheet.
Pulse almonds, 3 tablespoons sugar, and the flour in a food processor until ground to a coarse meal. Sprinkle almond mixture over dough. With a spatula, transfer plum slices to dough, spacing close together and leaving a 2-inch border; press lightly to fan out. Fold edge of dough over fruit. Refrigerate 30 minutes.
Brush dough with cream; sprinkle galette evenly with remaining 2 tablespoons sugar. Bake until crust is deep golden, and plums are juicy and bubbling, about 70 minutes. Transfer to a wire rack, and let cool completely.
