Cook the sweet potato in boiling water for 10 minutes or until tender, drain and mash, allow to cool slightly.
Meanwhile, place the fish in a shallow pan and just cover with water, poach for 5 minutes, until just cooked then drain and flake.
Mix the mash with the fish, mustard, oats, parsley and seasoning. Divide into 8 and mould into rounds.
Heat half the oil in a frying pan and fry 4 of the fishcakes for 3-4 minutes on each side until golden. Remove and keep warm. Repeat with the remaining fishcakes. Serve with salad.
Try adding some Cajun seasoning for a spicy variation. Potatoes work just as well as sweet potatoes and this is a great way to use up leftover mash.