Add the water, wine, cinnamon, and sugar to a saucepan over medium-high heat and bring to a boil. Turn off the heat, add the prunes, and let them soak in the liquid for 1 hour. Strain the prunes and discard the liquid.

Preheat the oven to 340°F.

In a food processor, combine all the ingredients for the crumble and pulse a few times, only until a sandy mixture forms. Leave the crumble mixture in the food processor bowl and refrigerate until ready to use.

Grease a 10-inch round cake pan (not a springform pan) with butter. Arrange the fresh plums in the pan, cut side down. Sprinkle the sugar evenly over the plums, then arrange the prunes in an even layer on top. Sprinkle the walnuts evenly over the prunes, followed by the crumble mixture.

Bake for 30 minutes, or until the sides are bubbling and the crumble is golden brown. Let cool for 10 minutes before serving. Serve warm with vanilla ice cream or whipped cream.