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Start by toasting the fennel, coriander and mustard seeds in a dry frying pan on a low heat, about 8-10 mins. Once you start to smell the rich aromas of the spices, you know they have cooked through. If they start to smoke, the spices will taste bitter and it would be better to start again. Once toasted, roughly crush using a pestle and mortar, or wrap the spices in a tea towel and crush with the base of a small saucepan. Stir in a pinch of black pepper.
Scrape the skin sides of the salmon with a knife to get rid of any remaining scales, and scrape off as much of the film of moisture you sometimes get (this helps to get the skin crisp). Slash the skin 3 times, about 3cm in length, to stop it shrinking during cooking.
Tip the spice mix onto a plate, lay the fish on it, skin-side up, so that the spices cover the underside of the fish. You can do this a few hrs ahead to help infuse the fish with the spices. You can also start the sauce ahead – put the finely chopped shallots in a pan with the vinegar, fish stock and saffron. Bring to the boil, then reduce by two-thirds. Set aside.
Put the potatoes in a pan and cover with salted water. Bring to the boil and boil for 1-2 mins only, then drain. Heat the oil in a large frying pan, then add the turmeric and chilli powder, if using. Toss the potatoes in the spiced oil, then cook for 5 mins without stirring. Shake the pan and cook for 5-10 mins more, or until the potatoes are cooked through. Season and keep warm.
When ready to cook the salmon, rub the skin sides with some oil, then fry in a pan (skin-sides down) on a medium heat with a little bit more oil, pressing down lightly on the fish for a few secs when it first goes in the pan. Don’t touch the fish or move the pan for about 5-6 mins – this ensures you get beautiful crisp skin. When the edges start to look golden, turn the fish over and cook for 1 more min.
While the fish is cooking, cook the spinach in a large pan of boiling water for 2 mins. Drain, then squeeze out the excess water. Return to the pan and stir in the butter. This will form a simple emulsion, which will prevent water running out over the plate. Season.
To finish the sauce, warm the pan of reduced mixture until hot. Take off the heat, then whisk in all the butter, reserving 2-3 cubes. Once the butter is amalgamated, stir in remaining cubes. Season with pepper and use immediately.
Place a spoonful of spinach in the middle of each plate, then lay the salmon on top. Scatter some potatoes and pour some sauce around the fish. Sprinkle a few sea salt flakes over the fish and serve.