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Heat oven to 220C/200C fan/ gas 7 and put the fish fillets in a shallow ovenproof dish so they fit quite snugly in a single layer. Mix 1 tbsp rapeseed oil with the thyme and garlic, spoon over the fish, then grate over the lemon zest. Bake for 10-12 mins until the fish is moist and flakes easily when tested.
Meanwhile, heat the remaining oil in a non-stick pan and fry the pepper and leeks for 5 mins until softened. Add the almonds and cook for 5 mins more. Tip in the tomato purée, 5 tbsp water, the bouillion powder and vinegar, and cook briefly to warm the mixture through.
Add the juice of up to half a lemon and blitz with a stick blender until it makes a thick, pesto-like sauce. Serve with the fish and the wilted spinach.