Peel the onions and cut the white one into fine cubes. Wash parsley, shake dry, pluck leaves from the stalks and chop. Cut the fish into cubes. Finely chop 2/3 of the fish in the universal chopper.
Knead the mashed and diced fish, egg, 4 tablespoons of breadcrumbs and parsley. Season with 1 level teaspoon of salt, pepper and lemon juice and chill. Clean the salad, wash and shake dry.
Wash, clean and slice the tomato. Cut red onion into rings. Wash cucumber and cut into slices. Form fish mixture into meatballs and turn in the remaining breadcrumbs. Heat the oil in a pan and fry the meatballs on each side for about 3 minutes.
Cut the bread roll open and cover with lettuce, tomato, tartar sauce, meatball, cucumber and onions. Sprinkle with lemon to taste.