Slice the Black Plum (5) into 1/4-inch thick slices.
In a food processor, add the All-Purpose Flour (1 2/3 cup), Salt (3/4 teaspoon), and Granulated Sugar (1 tablespoon) and pulse 3-4 times to combine.
Scatter the small pieces of Unsalted Butter (11 tablespoon) into the flour mixture and pulse 10 times in 1-second intervals. It will look like coarse crumbs.
In a measuring cup, mix the Apple Cider Vinegar (1 tablespoon) and Water (1/3 cup) together.
Add 5 to 6 tablespoons of the water & cider mixture and process until the dough comes together, about 7 to 10 seconds. If it doesn’t come together add more water, 1 tablespoon at a time, until it does.
Transfer the dough onto a sheet of plastic wrap and flatten into a 4-inch disk. Wrap it tightly in plastic and refrigerate for an hour (or overnight).
Preheat the oven to 375 degrees F (190 degrees C).
Take the pie dough from the fridge and let it sit at room temperature for 10 minutes.
Unwrap and transfer it onto a lightly floured kitchen counter. Roll it into a 12-inch circle that is 0.15-inch thick. Trim the edges to a clean circle with a pastry cutter.
Transfer the dough into a baking sheet lined with parchment paper. At this point, if your dough is too soft, feel free to place it in the fridge to cool for 5 to 10 minutes. This way it will be easier to work with.
Sprinkle the dough with Granulated Sugar (3 tablespoon) leaving 1-inch border from the outside.
Arrange the sliced plums overlapping onto each other on top of the dough, starting from the middle.
Sprinkle with Granulated Sugar (2 tablespoon).
Roll the outer edges of the dough towards the plums. Brush the edges with Butter (2 tablespoon).
Bake the galette for 20 minutes, then rotate the pan.
Bake for another 20 to 25 minutes.
Slice Black Plum (3) into 1/4-inch thick slices. Place them into a saucepan and add Granulated Sugar (1/2 cup) and let it sit on the counter for 10 minutes.
Heat the saucepan in medium heat or until all the sugar is fully melted, for 10-12 minutes.
Pass it through a strainer and discard the pulp. Place the glaze into a small bowl.
Remove the galette from the oven and let it cool for 5 minutes.
Spoon or brush the top of the plums with the some glaze. Cut galette into slices, serve and enjoy!