Clean and peel the celery, parsley roots, parsnips and potatoes and cut into 1 cm pieces. Cut into the bay leaf several times, finely dice the onion.
Steam the onions in the hot butter until translucent. Add the vegetable pieces and sauté for 3 minutes while stirring. Add the stock and bay leaves. Cover, bring to the boil and cook over medium heat for 25-30 minutes until the vegetables are soft.
Whip 100 g cream until stiff and chill. Remove the bay leaf, finely puree the soup with the rest of the cream. Season to taste with salt, a pinch of cayenne pepper and lemon juice.
Bring the soup to the boil again shortly before serving. Add the whipped cream and puree the soup until frothy.