Lay a piece of silver foil, large enough to wrap the salmon in a very loose parcel, on a baking tray.
If the salmon is to be served hot, grease the foil with the butter, or use the oil if serving the salmon cold.
Thinly slice the lemon and put about half on the foil.
Put the salmon on top, with a few slices of lemon inside and lay the rest of the lemon along the top of the salmon.
Season generously and wrap loosely, making sure the edges are securely folded together so that no steam can escape.
Put into a medium oven, Gas Mk 4,350F,180C, for 10 minutes per pound. If serving hot, take it out of the oven and let it stand for a further 10 minutes. If serving cold, leave the salmon in the foil for two to three hours.
To serve: Unwrap the fish and gently pull off the skin.