Peel and dice the onion and garlic. Peel the potatoes and parsley root and cut into large cubes. Heat the butter in a saucepan. Sauté the onion, garlic, potatoes and parsley root in it. Pour in the broth and simmer for about 20 minutes with the lid halfway on. Puree with the hand blender. Stir in the creme fraiche. Season the soup with salt, pepper and horseradish.
In the meantime, cut the smoked salmon into strips and drizzle with lemon juice. Pour hot soup into cups, sprinkle with salmon. Garnish with parsley or chives.