Preheat the slow cooker if necessary; see the manufacturer's instructions. Cut the rhubarb into 2.5 cm (1 inch) thick slices and add to the slow cooker pot with the still-frozen raspberries, the sugar and water. Cover with the lid and cook on low for 2–3 hours or until the rhubarb is just tender.
When almost ready to serve, heat the butter in a frying pan, add the almonds and crumble in the flapjacks. Fry, stirring, for 3–4 minutes or until hot and lightly browned. Spoon the fruit into bowls, sprinkle the crumble over the top and serve with thick cream.