The bird should be prepared the night before. Mix the butter with the salt and freshly ground black pepper, then season the cavity of the bird.
Rub the butter mix all over the turkey. Fold a large piece of greaseproof paper to double thickness and lay over the breast to protect it during the cooking.
Leave it in the fridge overnight.
On the day you wish to serve the bird heat the oven to 220C/430F/Gas 7.
Take the turkey out of the fridge and allow it to come to room temperature while the oven is heating up.
Put the onions in a large roasting tray. Put the turkey on a trivet or wire rack in the tray.
Pour one cup of boiling water into the cavity of the bird and seal with a skewer. Pour two cups of boiling water into the bottom of the tin, then cover the whole thing with two layers of foil, making sure it is well sealed around the edges.
Cook for 20 minutes, then reduce the temperature to 200C/400F/Gas 6.
After 1Â½ hours, remove the foil and greaseproof paper.
Cook for a further 40 minutes and don't open the oven door until the cooking time is up.
To test whether the turkey is cooked, insert a skewer or knife blade into the point where the thigh joins the breast - the juice should run clear.
If it is pink, cook it for another 20 minutes and test again. Leave the turkey to rest in a warm place for at least 15 minutes before carving.
Strain the juice from the bottom of the tin into a large jug - the fat will rise to the top, leaving the aromatic turkey and onion beneath.
Skim off the fat use the juices to make a gravy or serve as it is.