Make the tartare sauce. In a small bowl mix together the mayonnaise, crème fraîche, gherkin, capers, dill and lime rind and juice.
Cut the plaice into strips and dip each strip in the beaten egg. Mix together the polenta, Parmesan and cayenne, and coat the fish strips evenly in the mixture.
Heat a little oil in a frying pan. Add the plaice strips, in batches if necessary, and fry for 2–3 minutes, turning occasionally to prevent burning.
Serve the goujons in a soft roll with salad and a spoonful of tartare sauce.