Onions peel and chop finely. Heat the fat in a large pot. Fry the onions in it while stirring. Deglaze with approx. 1 3/4 l water, bring to the boil
Stir in the soup paste and let it simmer for 5-10 minutes while stirring. Refine soup with crème fraîche. Season to taste with salt and pepper. Heat up the crab and mussel meat only briefly. Garnish the soup with dill