Fillet the monkfish tail and remove and discard any dark meat. Cut each fillet into six slices about 5 mm/¼ inch wide, cutting at an angle to give slices about 10 cm/4 inches long. Then cut the salmon into similar-sized pieces, but cutting at an angle straight across the fillet. Butter a tray large enough to take the fish without overlapping and lay the slices in. Add the fish stock and the sherry. Cover with buttered paper or foil and bring the liquid up to trembling point. Transfer to the oven and poach gently for 2 minutes or until the fish is cooked. When it is cooked, remove the slices from the stock and cover and keep warm.
Set the stock over a high heat and reduce until syrupy. Roughly chop the herbs and mix them together.