Adjust an oven rack to the center position and heat the oven to 400°F. Combine the crushed amaretti and the flour and set aside.
Cut each plum in half, remove its pit, and cut the flesh into 1/4-inch-thick slices.
Cover a baking sheet, preferably without sides, with kitchen parchment. (If your baking sheet has sides, flip it over and use the back.)
On a lightly floured surface, roll the galette dough into a 15-inch round. Transfer the dough by folding it in half, picking it up, and unfolding it on the lined baking sheet. Sprinkle the crushed amaretti over the dough evenly, leaving a 2-inch border without crumbs. Arrange the plums over the crumbs in concentric circles, overlapping slightly. Lift the edges of the dough and fold them inward over the filling, pleating as you go, to create a folded-over border. Brush the border with the melted butter and sprinkle the entire galette with the sugar. Bake for 30 minutes. Remove from the oven and sprinkle the raspberries over the plums. Return the galette to the oven and bake until the crust is browned and the fruit is cooked and tender, another 15 minutes.
Slide the galette off the parchment and onto a cooling rack. Let cool for about 10 minutes before slicing. For a shinier filling, brush on the jam before slicing.