Peel the potatoes, wash them thoroughly and cook in boiling salted water for about 20 minutes. In the meantime, wash and dry lemon and grate the peel. Halve the lemon and squeeze the juice.
Wash the fish, pat dry, cut into 4 pieces and sprinkle with lemon juice. Wash and clean the cucumber and slice it into thin slices. Peel and finely chop the onion. Mix vinegar, salt, pepper and sugar, add 2 tablespoons of oil.
Mix cucumber, onion and vinaigrette. Wash parsley and dill, dab dry and chop finely. Mix lemon zest, parsley and dill (Gremolata). Season fish fillet with salt and pepper.
Heat 2 tablespoons of oil in a frying pan. Fry the fish for about 5 minutes on each side at medium heat. Heat the milk. Drain the potatoes, mash with a potato masher, add hot milk, mash again and season with salt and nutmeg.
Sprinkle Gremolata over the fish. Put cucumber salad on a plate and arrange fish on top. Garnish with dill. Add mashed potatoes.