Steam the sweet potatoes for 15 minutes with the skin or cook them in lightly boiling water. Peel and cut into 1 cm thick slices.
For the dressing, put all the ingredients in a well-sealed jar and shake vigorously.
Cut out the stalk of the black cabbage leaves. Cut the leaves into wide strips. Season with a little salt and add 2 tablespoons of the dressing. Press through with your hands and set aside for 5 minutes.
Cut the plums in half, remove the seeds and cut into wedges. Marinate plums and sweet potatoes (still slightly warm) with a little dressing and arrange on a flat plate. Spread black cabbage on top. Drizzle a little dressing over the salad.
Sprinkle the salad with cashew nuts and broccoli sprouts.