Preheat the oven to 160 degrees Celsius.
Wash the rhubarb but don't dry it, and cut into pieces about 2.5cm (1 inch) long.
Put rhubarb in a pan over low heat, add the apple concentrate and ginger and cover. The rhubarb will slowly stew in the steam and its own juices. Remove from the heat when the rhubarb is tender but still retains its shape - about 5-10 minutes. Divide among 6 individual ramekins and set aside.
Coarsely chop the roasted nuts and mix with the oats and seeds.
Melt the barley malt in a saucepan with the corn oil and cinnamon and mix with a wooden spoon.
Pour the oil and malt mixture over the nuts and mix until thoroughly combined. Spoon the crumble mixture over the rhubarb and bake for 15 minutes.