Toast all the spices for the spiced salt in a dry pan for 2-3 mins until fragrant, then grind with the salt and lemon zest to a coarse powder using a pestle and mortar. Sprinkle over both sides of the fish. Put on a plate, cover and chill for 1 hr.
Heat the oven to 150C/130C fan/gas 2. Rinse the salmon well and pat dry with kitchen paper. Lay the salmon in a roasting tin that fits it snugly, then pour over the 350ml olive oil. Cook for 25 mins until the salmon is still pink in the middle but beginning to flake easily.
While the salmon is cooking, make the tahini sauce by whisking all the ingredients together in a bowl. In a second bowl, make the topping by mixing all the ingredients to loosely bind.
When you are ready to serve, carefully lift the salmon out of the olive oil and lay on a serving platter. Spoon the tahini sauce over the fish, smooth over, then press the pistachio topping over the surface. Serve immediately.
