You can make this with all sorts of other fruits. Apples, pears, plums and quinces work well. If you use quinces, you need to cook them until tender before topping with crumble mixture.
Preheat oven to 200°C/180°C fan-forced. Combine rhubarb, frozen raspberries and strawberries in a large oven-proof dish.
Place flour, cinnamon and butter in a food processor and pulse until it has a crumb-like consistency. Stir in the sugar, chopped almonds and rolled oats.
Sprinkle crumble over the rhubarb mixture and bake for 35-40 minutes. Cool for 10 minutes and serve with vanilla ice-cream or custard.