Wash the fish and clean the inside thoroughly. With a very sharp knife cut through the skin, right round the fish, ½ in. (1 cm) from the edge. Be careful to cut right through and make sure the side cuts join up at the tail or you will be in trouble later on. Sprinkle the fish with salt and pepper and lay it in ¼ in. (6 mm) water in a shallow baking tin. Bake in the oven at gas mark 6, 400°F (200°C) for 20 to 30 minutes, according to the size of the fish. The water should have just evaporated as the fish is cooked.
Meanwhile, melt the butter and stir in the herbs. Just before serving, pull the skin off the fish (it will tear badly if not properly cut) and spoon over the butter.