Mix together flour, sugar, and salt in large bowl.
Add cubed butter with a pastry cutter until mixture is crumbly.
Add sour cream and mix together.
Gather dough into a ball, flatten into a disc shape, wrap in plastic wrap and chill for at least an hour before rolling out.
For the Filling:
In a medium bowl mix the cinnamon, lemon juice, orange/lemon zest, vanilla, and flour/cornstarch, then add the sliced plums.
Prepare the Galette
Preheat oven to 375.
Remove the pie dough from refrigerator. Let stand for 5 minutes. Roll out the dough on a lightly floured surface to about 13 inches around and as even as possible.
Place dough on parchment paper.
Arrange plum slices in center of dough leaving about a 2 inches of dough uncovered around the edges. When all the plums are arranged, fold the empty dough up toward the center.
Optional: brush with heavy cream and sprinkle with course sugar.
Bake at 375 degrees for 40-50 minutes until crust is golden brown. Let cool slightly before serving.
Notes
Note: For the edges of the crust: if you don't have course sugar, regular sugar works as well or you can just brush with cream and leave the sugar off completely as I did.