Sauté the onion and garlic in 1 tbsp. butter until soft. Add the finely chopped vegetables and mix well. Extinguish with the fish stock and simmer on low heat until the vegetables are soft.
Mix the remaining butter well with flour (flour butter) and add to the soup. When the soup is creamy, reduce the heat and fold in the whipped cream. Add the fish pieces and simmer in the soup for 4 minutes. Season to taste with salt and freshly ground pepper. Sprinkle with parsley before serving.