Preheat the oven to 60°C, warm the platter and plates.
Peel and chop the potatoes. Separate the cauliflower into florets. Cook the potatoes and cauliflower together in boiling salted water for approx. 30 mins. until very soft. Drain, leave in a colander for approx. 10 mins. to allow the residual moisture to evaporate, return to the pan. Mash the potatoes and cauliflower using a potato masher or fork.
Heat the oil in a non-stick frying pan. Season the salmon, fry for approx. 7 mins. on each side, transfer to a plate. Pull apart the salmon using two forks, add to the mash, mix.
Add a little grated lemon zest, squeeze out 1 tbsp of juice, mix into the mash along with the potato starch, polenta, salt and pepper. With wet hands, shape the mixture into approx. 12 patties. Heat the oil in a non-stick frying pan. Fry the patties in batches for approx. 5 mins. on each side, remove, keep warm.
Finely chop the herbs, mix with the yoghurt and mayonnaise, season. Serve the dip with the patties.