Mix 4 tablespoons of lemon juice with water. Wash and peel the salsify (ideally with household gloves) and cut into pieces approx. 3 cm long. Immediately put these in the lemon water. Peel the onion and chop finely.
Heat the butter in a saucepan and sauté the onion until translucent. Drain the salsify (save a few pieces for garnish later), add the vegetable broth and simmer for about 15 minutes until the salsify is soft. Add milk and crème frâiche and puree everything finely. Heat again and season to taste with the remaining 2 tablespoons of lemon juice, salt, pepper and a pinch of nutmeg and sugar each. Garnish with the walnuts and the remaining salsify.