Cook the potatoes in plenty of boiling water for 15-20 minutes. Then rinse with cold water, peel and leave to rest. Meanwhile, hard-boil eggs for 8-10 minutes. Rinse cold and peel.
Clean and wash the radicchio and cut into bite-sized pieces. Roughly chop the walnut halves. Wash the dill and pluck the little flags. Mix mustard and sour cream for the sauce. Season with salt and pepper. Cut potatoes and eggs into slices.
Heat the vinegar and stock. Pour over the potato slices. Mix carefully with the remaining ingredients. Drizzle with the sauce and serve.