In a food processor, add the 1 1/4 cups of flour, sugar, the salt and pulse to combine. Add the butter and pulse until the mixture resembles coarse meal. Add sour cream and the ice water and pulse until the dough just barely comes together. Gather the dough and pat it into a disk. Wrap the dough in plastic and refrigerate until chilled, about 30 minutes.

Preheat the oven to 425 F degrees. Line a baking sheet with parchment paper.

Working on a lightly floured surface, roll out the disk of dough to a 12-inch round; transfer to the baking sheet.

In a bowl stir the brown sugar with the cinnamon together.

Arrange the plums in the center of the dough, leaving a 1 1/2-inch border all around. Sprinkle with the sugar mix evenly over the plums. Fold the edge of the dough up and over the plums. Brush the rim with the egg wash and sprinkle with additional brown sugar if preferred.

Bake the galette for about 45 minutes, or until the crust is golden and the fruit is tender and bubbling. Let the galette cool on the baking sheet for 30 minutes, sprinkle with some powdered sugar, then cut into wedges and serve.