Onion peel, in fine cubes cut. Heat the oil in a pot. Sauté onion in it. Peel potatoes, cut into cubes and add to the onion. Bring to the boil with broth and simmer for about 20 minutes.
Add milk and water, stir in and puree everything finely.
Clean and wash spring onions and cut into fine rings. Dill wash, shake dry, pluck flags from the stems and chop. Pour soup into 8 bowls. Garnish with spring onion rings, crabs and dill.
Add 1/2 tablespoon sour cream to each and serve.