How to Make It
Heat oven to 350° F. Mix together the flour, oats, butter, brown sugar, cinnamon, and ¼ teaspoon of the salt in a medium bowl. Transfer to a rimmed baking sheet and, using your hands, form the mixture into large clumps. Freeze until hard, 15 to 20 minutes.
Bake the crumble until golden brown, 20 to 25 minutes. Let cool.
Meanwhile, mash the blackberries and granulated sugar in a medium bowl. Add the plums, lemon juice, and the remaining pinch of salt and mix to combine.
Divide the ice cream, fruit and juices, and crumble among 4 glasses or bowls. Serve immediately.