To make the fish cake, place the mashed potato, smoked trout, chilli flakes, coriander and egg into a large bowl and mix together well. Season the mixture well with salt and freshly ground black pepper and shape into a ball.
Roll the fishcake in the sesame seeds until coated, then flatten to a patty shape.
Heat the oil in a frying pan over a medium heat. Add the fishcake and fry for three minutes on each side, until golden and cooked through.
To make the dressing, place the capers, yoghurt, chives and lime juice into a bowl. Season, to taste, with salt and freshly ground black pepper.
To serve, place the fishcake in the centre of a warm plate, spoon some of the dressing around the plate and sprinkle over the chopped coriander.