Preheat oven to 50°C. Cook potato/kumara in a saucepan of boiling water until soft, 12-15 minutes. Drain well, return to the pan and place back onto low heat for a few minutes – this will help the potato/kumara dry out a bit. Mash roughly, then mix well with salmon, spring onion, parsley, egg, lemon zest and salt and pepper.
Scoop out 1/3 cup portions of fishcake mixture and shape into patties. Carefully dust each patty in flour.
Heat a good drizzle of oil in a large non-stick frying pan over medium heat. Cook fishcakes in batches for about 3 minutes each side until golden brown. Wipe the pan clean with a paper towel in between batches and add a little more oil as needed. Keep fishcakes warm in the oven as you cook them.
Mix all lemon caper mayo ingredients together.
To serve: place 2-3 fishcakes on each plate and top with a poached egg (if desired). Serve with watercress or rocket, a dollop of lemon caper mayo, and a lemon wedge on the side.
Tip – If cooking the potato/kumara from scratch, don’t skip the step of drying the it out over a low heat – this will ensure the mixture is not too wet. Once you’ve cooked the fishcakes, you can freeze them (they will keep for a couple of months). Then just defrost and reheat in the oven until hot through for a convenient meal.