Wash the fish, dab dry and cut into large cubes. Sprinkle with lemon juice. Heat the fat in a pot and sauté the flour in it while stirring. Deglaze with stock and milk, bring to the boil and simmer for five minutes, stirring occasionally.
Stir in mustard, season with salt and set aside. Heat two tablespoons of oil in a frying pan, drain the fish and put it in the hot oil. Fry for five minutes while turning over, until golden brown. Season with salt and remove from the pan.
Add the remaining oil to the pan, heat it up and fry the potatoes for eight to ten minutes, turning them over. Clean, wash and cut the leek into rings. Two to three minutes before the end of the frying time, add to the potatoes and fry.
Season potatoes with salt and pepper. Put the fish in the pan and fold into the potatoes. Pour mustard sauce over it and heat everything up again briefly. Cut two gherkins into a fan shape and garnish the pan with the gherkins, dill and parsley.