Clean the leek and cut into fine rings. Sauté in the melted butter. Now deglaze with the broth and bring to the boil. Simmer for 10 minutes, then remove 4 tablespoons of leek and set aside.
Puree the rest and stir in 2/3 of the cheese with the crème fraiche. Season to taste with the spices. Add the prawns and the rest of the leek to the soup and heat.
Arrange on plates and sprinkle with the rest of the cheese.