Peel the onions and finely dice the white one. Wash parsley and shake dry. Pluck leaves from the stalks and chop finely.
Wash the fish, dab dry. Remove any bones with tweezers. Dice 2/3 coarsely, chop briefly in a universal chopper or chop very finely with a kitchen knife. Finely dice the rest.
Knead finely chopped and diced fish, diced onion, parsley, 4 tbsp. breadcrumbs and egg. Season with pepper, 1 level tsp. salt and lemon juice. Put the mixture in a cool place.
Cut red onion into rings. Wash the salad and dab dry. Wash tomato and cucumber and cut into thin slices. Form 4 flat meatballs from the fish mixture with moistened hands.
Turn the meatballs in the remaining breadcrumbs, press them slightly. Heat the oil in a pan. Fry the meatballs for 4-5 minutes on each side. Do not turn until a crust has formed at the bottom.
Cut open the buns. Spread the lower halves with remoulade. Cover each half with lettuce, tomato, meatball, cucumber and onion rings. Put the lid of the roll on top. Serve with lemon wedges.