Fillet the fish, add boneless cuts, wash and dry with paper towels. Chop the fish finely together with the parsley, add the egg, breadcrumbs and salt. Mix well and form a square cutlet. Put it in the fridge to solidify..
Heat a generous portion of oil in a frying pan. Remove the cutlet from the fridge and coat it in whisked egg and breadcrumbs - twice. Fry the turbot chops until golden brown on both sides (2-3 minutes on each side). Cut the roll in half and spread each half with tartar sauce. Place lettuce on the sauce and arrange the fried turbot on the lettuce. Add the tomato and onion and put the sandwich together..