Make the dough: In a large bowl, add the flour, sugar, and salt; stir to combine. Add half the cold butter and use your fingertips to work it into the dry ingredients, pinching the butter pieces until the mixture has the texture of coarse oatmeal. Add the remaining cold butter and quickly work it into the dough until the biggest clumps of dough are the size of large lima beans. Dribble about ½ cup ice water into the dough, a little at a time, tossing and mixing between additions. (Don’t try to dampen all of the dough evenly. It should look rather ropy and rough.) Stop adding water when there are still a few bits of dry flour remaining in the bottom of the bowl. Divide the dough into 2 equal pieces and flatten each into a 1-inch-thick disk. Wrap each disk tightly with plastic and refrigerate for at least 2 hours or up to 3 days.
Make the galette: Place a rack in the lower third of the oven and preheat to 400°F.
Lightly flour a large sheet of parchment paper and a rolling pin. Retrieve and unwrap 1 disk of dough (reserve the remaining disk for another use) on the prepared parchment. Roll the dough out to an even 14-inch round. In a large bowl, combine the flour, almonds, and 2 tablespoons sugar. Sprinkle the mixture evenly over the dough, leaving a 2-inch border. Arrange the plums over the filling in barely touching concentric circles, again leaving the 2-inch border. Sprinkle the plums evenly with 2–3 tablespoons sugar, depending on the sweetness of the fruit. Trim away most of the 2-inch border, leaving about ½ inch of dough. Fold the exposed border over on itself, crimping to make a narrow rim around the plums. Brush the rim generously with the melted butter, and sprinkle lightly with more sugar. Use the parchment to carefully slide the galette onto a large rimmed baking sheet. Freeze until the outer rim of dough is firm to the touch, about 10 minutes.
Bake until the plums are tender, the juices are thickened and bubbly, and the crust is well-browned and caramelized at the edges, 40–45 minutes. Let cool on the baking sheet for 20 minutes. Using a thin offset spatula, loosen the galette from the parchment and carefully slide onto a cake stand or large flat plate. Glaze with jam, if desired, and serve warm.