Peel and finely chop the onion. Peel, wash and cut the potatoes into small pieces. Melt butter in a pot. Sauté potatoes and onion. Add stock and vinegar.
Bring to the boil, cover and cook for about 20 minutes. In the meantime, mix horseradish and 3 tablespoons of cream until smooth. Cut salmon into pieces. Wash dill, dab dry, pluck flags from stems and put some aside for garnishing.
Coarsely chop the remaining dill, add to the soup and puree everything finely. Stir in 100 g cream and season to taste with salt, pepper and honey. Add the smoked salmon to the soup and bring to the boil again. Serve the soup, add horseradish cream in streaks and sprinkle with remaining dill.