For the fish fillet with lemon sauce, wash the potatoes. Boil in water for about 25 minutes, then rinse and peel.
Wash and slice the tomatoes and cucumber. Peel and dice the onion. Cut potatoes into slices. Bring soup, vinegar, onion, salt and pepper to a boil and pour over. Fold in cucumber and tomatoes.
Wash fish and cut into portion pieces. Sprinkle with 1 tablespoon lemon juice and salt. Beat the egg. Dredge the fish first in flour, then in egg. Fry in hot oil for 4-5 minutes on each side.
Season sour cream with the rest of lemon juice, salt and pepper. Sprinkle potato salad with chives and serve the fish fillet with lemon sauce right away.