Preheat the oven to 200°C.
Boil the potatoes in a large pan of salted water for 15-20 minutes until tender.
Drain potatoes, crush slightly with a fork, season, and drizzle with oil.
Roast potatoes in the oven for 25-30 minutes until golden-brown and crisp.
Combine salmon, crab meat, mayonnaise, herbs, chilli, and mustard in a bowl.
Shape the mixture into cakes and chill in the fridge.
Mix all tartare sauce ingredients together in a bowl and season to taste.
Fry fishcakes in oil over medium-high heat for 3-4 minutes on each side.
Wilt the spinach in a small frying pan for 1-2 minutes and season with oil.
Divide fishcakes, potatoes, and spinach between plates and serve with tartare sauce.