Prepare the plums: remove the stones and chop/ slice the plums into smaller pieces. Place in a large bowl and add the sugar, ground cardamom and cinnamon. Mix well to combine and set aside for 20 minutes.
Preheat the oven to 175°C (160°C fan). Line the large baking sheet with baking parchment. Drain the plums from their juices (save the juices and set aside), and mix them with chopped walnuts and breadcrumbs.
Cut a piece of parchment paper larger than the filo pastry sheets and place on a clean working surface. Add the first sheet of filo pastry on top of the parchment paper. Using a pastry or silicone brush, brush the first sheet of filo with the melted butter. Lightly dust it with icing sugar, then add another sheet of pastry on top. Gently press the sheets of pastry together, then repeat the process with the remaining sheets of filo (7 sheets in total). Brush the top layer with melted butter and dust with icing sugar.
Pile the filling along the length of the pastry along one side (about 2 inches from the edges). Using the parchment paper underneath to help you, roll the pastry up to enclose the filling. Tuck the ends in and roll seam-side down onto the lined baking sheet. Brush the top with the reserved juices and any remaining melted butter.
Bake 35- 40 minutes until golden brown. Leave to cool for 10- 15 minutes and dust with icing sugar. Slice and serve with ice cream, cream, or custard.