Put half the butter, some onion slices, bay leaves. lemon slices and tarragon sprigs into the centre cavity of the fish along with a good seasoning of salt & pepper; spread the rest of the butter over the skin of the fish.

Wrap the foil over the salmon to make a loose but tightly sealed parcel. Place the fish on a large baking sheet or roasting tray (if needed, place diagonally so that it fits in the oven). If the fish is too big for your oven, simply bend the tail end upwards. Bake in the centre of the oven for 2½ hours at gas mark 3 or 160C.

After cooking remove the salmon from the oven and allow to cool completely before serving (It's best not to open the foil). To serve, remove the skin - this will come off very easily if you make a horizontal slit all along the middle of the side of the salmon. Ease the fillets away from the bone and serve.