To make the dressing, place the garlic and salt in a mortar and crush well. Stir the garlic, yoghurt and tahini together until it becomes a thick paste. Thin slightly with lemon juice (it should be the consistency of pure cream). Taste for salt and refrigerate.
Preheat the oven to 150°C Place the salmon on a large sheet of baking paper. Season all over with salt and pepper and drizzle with 2 ½ tablespoons of the oil. Wrap the salmon in the paper, place on a tray and bake in the centre of the oven for 25 minutes. Gently turn the salmon over and cook for a further 25 minutes, by which time it should be medium–rare. Remove from the oven and open the paper to stop the salmon cooking further. Leave to cool to room temperature.
Turn the oven up to 200°C and roast the walnuts for 5 minutes. Remove from the oven and rub briskly in a clean tea towel to remove as much of their skin as possible. Chop finely and set aside.
Close to serving time, use a sharp knife to cut through the skin along the back of the salmon from head to tail. Peel away the skin from one side. Scrape away the thin layer of grey flesh (the blood line) to expose the pink flesh underneath. Carefully transfer the salmon to a large serving plate. Brush ½ cup of the yoghurt dressing over the top of the salmon.
In a bowl, whisk the remaining oil with the lemon juice, salt and pepper. Add the onion and chilli and whisk lightly. Add the chopped walnuts, herbs and sumac. Spoon the salad over the salmon, covering it as neatly as possible.
Use a metal spoon to ‘cut’ portions of the salmon and salad. Serve with the remaining dressing.