Preheat the oven to 425 degrees F (220 degrees C) and lightly grease a 9-inch springform pan. Set aside.
In a large bowl, combine the sliced plums with the granulated sugar. Set aside to allow the fruit to release juices.
In a medium bowl, mix the flour, salt, brown sugar, and cornstarch. Add the diced vegan butter and use a pastry cutter or fork to blend until the texture is sandy. Set aside.
Arrange the plums in a single layer on a parchment-lined baking sheet. Bake for 10 minutes, then remove and let cool completely. (This helps draw out moisture and intensify flavor.)
Gently press the pie dough into the prepared springform pan. Let the excess dough hang over the sides for now; it will be folded later.
Sprinkle the flour mixture evenly over the bottom of the pie crust.
Neatly layer the cooled plums over the flour mixture. Fill in any gaps with leftover slices to ensure the top is well covered.
Fold the overhanging dough inward to create a rustic edge that holds in the filling.
Bake for 35 to 40 minutes, or until the crust is golden and the plums are bubbling.
Let the tart cool completely before removing from the pan.
Slice and serve.
Notes
Would go well with some vegan vanilla ice cream to balance out the sourness of the plums.
Use ripe but firm plums: Overripe plums release too much juice, make the crust soggy, and may become mushy.
Slice the plums evenly: To ensure even baking.
To prevent a soggy crust: Don’t skip the flour filling. It soaks up juices.
Sweeten to taste: Based on the tartness of the plums.
Use chilled dough: To ensure the best, flaky, crisp pastry.