For the shortcrust pastry, quickly knead all the ingredients and refrigerate for 30 minutes. Then roll out the dough and lay out the bottom and edge (3 cm high) of a greased springform pan or tart pan (28 cm).
Prick the bottom several times with a fork and distribute the semolina evenly on top. It soaks up the juice of the plums so that the base stays crispy.
Wash and stone the plums and divide them in quarters upright on the dough.
Melt the butter. Mix the flour, sugar and the chopped or grated walnuts, pour in the melted butter while stirring. Let the streusel cool down briefly and place on the plum cake.
Bake in the preheated oven on the second rack from below at 180 ° C for about 40 minutes.