Peel onion, chop coarsely. Peel, wash and chop the potatoes a little bit smaller. Heat the oil in a large pot. Fry the onion in it. Add the potatoes and steam briefly. Deglaze with fish stock and 1⁄2 l water. Stir in broth, bring to the boil. Cover and simmer for about 25 minutes. Clean, wash and cut leek into rings. Add after about 10 minutes cooking time.
For the Gremolata Dill wash, shake dry and chop finely. Peel garlic and chop finely as well. Mix dill, garlic and lemon peel.
Rinse fish, dab dry and cut into cubes of approx. 2 cm. Finely puree the soup with a hand blender. Season to taste with salt, pepper and lemon juice. Add the fish to the soup and let it simmer for about 5 minutes.
Stir sour cream into the soup and season to taste again. Serve and sprinkle with dillgremolata. Serve with brown bread.