Preheat the oven to gas 6, 200°C, fan 180°C. Lightly grease a baking tray with the olive oil (check the tray is large enough for the salmon). Arrange a layer of herbs and lemon slices on it. Using a sharp knife, make small slashes across the fish, about 3cm (1¼in) apart. Then season well all over.
Put the fish on top of the herbs and lemon. Put some lemon slices inside the fish and cover the top with a few more lemon slices. Drizzle the remaining oil over the fish. Bake in the oven for 35 minutes or until just cooked through. Insert a metal skewer into the thick end of the fish; if it’s cooked, the skewer should come out hot.
Meanwhile, make the sauce. Heat the oil in a small pan. Add the onion and cook over a low heat for 7 minutes until softened. Then pour over the stock, add the lemon zest and juice and simmer for 5 minutes. When the fish is ready, stir in the mustard, honey and dill, and season to taste.
To serve, remove the skin if you like. Then slice the fish into portion-sized fillets and use the flat of the knife to work along each piece, easing it away from the bone. Carefully turn over and repeat. Serve drizzled with the mustard dill sauce.
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