Preheat the oven to 220°C fan.
Pat dry the salmon and sprinkle with salt and pepper. In a small bowl, combine the za’atar and sumac, then sprinkle this all over the top of the salmon to create a crust.
Place a large ovenproof sauté pan on a medium-high heat and add a tablespoon of oil. Once hot, add the spinach and a pinch each of salt and pepper and cook for 2-3 minutes, until just wilted.
Top with the salmon, skin side down, and drizzle the top of the fish with 2 tbsp of oil. Bake for 5 minutes.
Meanwhile, in a small bowl whisk together the tahini, garlic, 2.5 tbsp of lemon juice, a good pinch of salt and 100ml of water until smooth and quite runny.
When ready, remove the pan from the oven and pour the tahini all around the salmon (but not on the fish at all). Bake for another 5 minutes, or until the fish is cooked through and the tahini is bubbling. Spoon over the remaining tbsp each of lemon juice and oil and top with the coriander and extra sumac.