I started with this recipe made some modifications.  I found the original not to be tart enough, hence the pomegranate molasses.

Here is the recipe at Food Network

Ingredients:

• filling:
  • ¾ pound Italian prune plums, quartered and pitted
  • 2 tablespoons Minute tapioca (sub 1 T cornstarch)
  • 2 tablespoons creme de cassis liqueur
  • plus some of the sugar below
  • 1 ¾ cups sugar, divided (sub some pomegranate molasses)
• crust:
  • plus some of the above sugar
  • ¼ pound (1 stick) unsalted butter, at room temperature
  • 1 ¼ cups all-purpose flour
  • ½ teaspoon ground cinnamon (sub cardamom?  )
  • ½ teaspoon kosher salt
  • ¼ teaspoon baking powder
  • (add orange zest?  ) (juice?  )

Directions:

• soften butter
• Preheat the oven to 350 degrees.
• Butter and flour a 9-inch spring-form pan and place it on a sheet pan.
• Place the plums, tapioca, creme de cassis, and ¾ cup of the sugar in a mixing bowl and stir to combine.
  • Allow to sit for 15 minutes.
• In a small bowl, combine the flour, cinnamon, salt, and baking powder.
• In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and remaining 1 cup of sugar on medium speed until light and fluffy.
  • With the mixer on low, gradually add the dry ingredients to the butter mixture until it forms small, dry crumbs.
  • Add 1 tablespoon of cold water and continue to beat for about 30 seconds, until the mixture forms large, moist crumbs.
• Set aside ¾ cup of the crumb mixture and pour the rest into the spring-form pan.
  • With floured hands, lightly pat the dough evenly in the bottom of the pan and 1 inch up the sides.
  • Arrange the plums in concentric circles on the crust.
  • Sprinkle the remaining crumb mixture evenly on top.
• Bake for 1 hour, until the fruit is bubbling and the crust is golden.
• Cool for 15 minutes, then remove the sides of the pan and serve warm or at room temperature.

My spring-form is 10 inches and some other adjustments:

• filling:
  • 1 lb of plums
  • 4 T c de c
  • 4 T pom molasses
  • 1 T cornstarch
  • ½ cup sugar
  • blood orange zest
• crust:
  • 1 ⅓ C flour
  • ¾ t cardamom
  • ¾ C sugar
  • I used juice from the plum mixture instead of water (and it was more like 2 T)