Peel and dice shallot. Peel and dice the celery and potatoes.
Steam the diced shallots in half of the melted butter until translucent. Add celery and potatoes and sauté for about 2 minutes. Pour in the broth and let everything simmer covered for about 15 minutes. Stir in the cream and crème fraiche. Puree the soup finely and bring to the boil again briefly. Season to taste with salt, pepper and nutmeg. Finally stir in the sparkling wine and keep the soup covered and warm.
Clean the mushrooms, cut them into bite-sized pieces and fry them in the remaining melted butter. Salt and pepper. Clean and finely chop the chervil.
Spread the mushrooms on plates and pour the hot soup over them. Serve sprinkled with chervil.
Variations are possible by using mushrooms or forest mushrooms or sprinkling with small croutons.