Prepare the fishcakes in advance and chill. Remove from the fridge 30 minutes before cooking, and fry just before serving.
Preheat the oven to 200C/400F/Gas 6.
For the potatoes, boil the potatoes in a large pan of salted water for 15-20 minutes, or until tender. Drain, and then return to the saucepan.
Crush the potatoes slightly with a fork, season, to taste, with salt and freshly ground black pepper, and drizzle over the oil. Tip into a roasting tin and roast in the oven for 25-30 minutes, or until golden-brown and crisp.
Meanwhile, for the fishcakes, combine the salmon, white and brown crab meat, mayonnaise, parsley, dill, red chilli and Dijon mustard in a bowl. Season, to taste, with salt and freshly ground black pepper.
Shape the mixture into cakes using your hands, and chill in the fridge until the potatoes are almost ready.
For the tartare sauce, mix all of the ingredients together in a bowl and season, to taste, with salt and freshly ground black pepper. Set aside.
To cook the fishcakes, heat the oil a large frying pan over a medium-high heat. Fry the fishcakes in the oil for 3-4 minutes on each side, or until golden-brown and crisp.
For the spinach, place the spinach into a small frying pan over a medium heat and cook for 1-2 minutes, or until wilted. Drizzle over the olive oil and season, to taste, with salt and freshly ground black pepper.
To serve, divide the fishcakes between two serving plates, place the roasted potatoes and spinach either side, and spoon the tartare sauce around the edge of the plate.