In the bowl of a food processor, combine the flour, butter and salt. Process until the mixture resembles coarse meal, about 8 to 10 seconds. With the machine running, add ice water in a slow, steady stream through the feed tube. Pulse until the dough holds together without being wet or sticky. Squeeze a small amount together and if the dough is crumbly, add more ice water, 1 tablespoon at a time.

Remove the dough from the food processor and gather it into a ball. On a lightly floured surface, roll out the dough into a 16-by-18-inch oval 1/16 to 1/8 inch in thickness. Using the rolling pin, drape the dough over the pin and transfer it to a rimmed baking sheet lined with parchment paper or a silicon baking pad. Chill the dough until firm, about 20 minutes.

Preheat the oven to 400° F. Place the sliced plums in a large bowl and add the flour, sugar, butter pieces, vanilla and lemon juice. Gently toss to combine. Spread this mixture evenly over the dough to within 2-3 inches of the edge. Fold the edge of the dough up over the plums to create a 2-inch border. Sprinkle the slivered almonds over the top of the plum filling. This step is optional; brush the pastry dough with the egg wash to promote browning and then sprinkle the crust with a bit of sugar.

Bake the galette on the middle rack in the oven for about 50-60 minutes, until the fruit is very soft and the crust is nicely browned. Let the galette cool to room temperature before serving.