Prepare your Idahoan Butter & Herb Perfect Mash to packet instructions. Gently mix together with the fish, 150g flour and the chopped parsley. Season with salt and pepper. Chill in the fridge for 1 hour 30 minutes. Add the remaining flour and shape into 6 patties.
Meanwhile, you can make the tartare sauce. Mix together all the tartare sauce ingredients and season with salt and pepper. Leave in the fridge until using.
Heat half of the oil in a large frying pan and cook the fishcakes (three at a time) for 8 minutes, turning once, until they are golden and crispy on the outside. Serve with a spoonful of tartare sauce, a wedge of lemon and some watercress leaves.