Grilled herrings with mustard sauce

• 4 fine fresh herrings
• oil
• salt

Scale and clean the herrings and wipe them with kitchen paper. Work all the ingredients for the sauce together into a smooth paste, in the top of a double boiler. Stir gently over hot water until the sauce thickens, but do not allow the water to boil. Keep the sauce warm while you grill the herrings. On each side of the fish make two or three diagonal incisions about 4 cm (1½ in) apart, cutting