To make the crust, place the flour, sugar and salt and a food processor, and process to blend. Add the butter and pulse until the mixture resembles coarse meal. Add 3 tablespoons of the cold cream and pulse until the dough comes together in a ball. Add some or all of the remaining tablespoon of cream if the dough is still quite crumbly; you need to be able to squeeze it so it holds together in a ball.
Place a large piece of plastic wrap on the counter, gather the dough into a ball, and flatten the dough into a disk on the plastic wrap. Wrap well. Refrigerate for 1 hour.
Preheat the oven to 400°F . Place a large sheet of parchment paper on a work surface, and lightly flour the parchment. Place the dough on the paper, lightly flour the top, and roll out the dough into a round or oval about 1/4-inch thick. Transfer the paper to a rimmed baking sheet.
Toss the plums with 4 tablespoons of the sugar, the flour, cinnamon, and the crystallized ginger. Arrange the plums in concentric circles on the sprinkled area of the dough, and then sprinkle the blueberries and ginger over and in between the plums. Fold the border in over the fruit. Sprinkle the border with the remaining tablespoon of sugar.
Bake for 40 to 45 minutes, until the crust is nicely browned and the fruit filling is soft and a bit bubbling.
Warm the preserves in the microwave, and gently brush the hot galette with the preserves, both the top of the crust edges, and the fruit in the middle. Transfer the parchment paper with the galette to a wire rack and let the galette cool to room temperature. Transfer to a plate and serve, with whipped cream if desired.
Notes
Sometimes I peel stone fruit before using it in baked desserts, and sometimes I don’t. You can do this by dropping the fruit in a pot of boiling water for about 20 seconds or so, then taking them out with a slotted spoon and transferring them to a waiting ice water bath. After a minute in the ice water, remove the fruit and ideally the skin should peel off easily.