For the fruit, in a large bowl, combine the plums, brown sugar, flour, and cassis. Pour the mixture into a 12 by 8-inch shallow baking dish.
For the topping, combine the flour, granulated sugar, brown sugar, salt, oatmeal, walnuts, and butter in the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed until the mixture is crumbly and the butter is the size of peas. Scatter evenly over the plum mixture.
Bake the plum crunch for 40 to 45 minutes, until the plums are bubbling and the top is browned. Serve warm or at room temperature with ice cream.
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