Have the fish cleaned but left whole. Place in the centre of a large square of foil – buttered for serving hot and oiled for serving cold. Arrange the lemon slices and parsley stalks over the fish. Fold the foil over the top and turn up the ends to wrap the fish completely in a loose foil parcel. Place in a large roasting tin or on a baking tray. Set in the centre of a moderate oven (180°C., 350°F., Gas Mark 4) and bake allowing 15 minutes per
To serve hot, let the baked trout stand in the wrapped package for 5 minutes to allow the flesh to settle. To serve cold, leave the fish wrapped in the packet until quite cold.
Open the foil, discard the lemon and parsley and lift the skin away from the fish. Separate the flesh down the centre following the line of the bone. Serve the flesh in pieces, lifting the centre bone away to get at the flesh underneath.