To make brine, place all ingredients except cold water in a very large, heatproof pot or bowl and stir to dissolve salt and sugar.

Add cold water, then turkey, reserving neck and giblets for gravy.

Cover and chill for 12-24 hours.

Preheat oven to 160˚C fanbake. Lift turkey out of brine and pat dry inside and out. Tie legs loosely, tuck wings under and place in a large roasting dish.

Rub skin all over with butter or brush with oil.

Cook for 2 hours at 160˚C, then increase heat to 200˚C and cook until a cooking thermometer inserted into the thigh shows 75˚C, or the juices run clear when a skewer is inserted into the thigh (about 30 minutes). If it starts to brown too quickly, cover those parts loosely with tin foil.

Transfer cooked turkey to a serving platter breast-side down to allow the juices to run back into the breast, cover with tin foil and a teatowel and rest for 15-30 minutes while cooking any vegetables and/or making the gravy.

While turkey is brining or cooking, prepare the gravy by putting the neck and giblets in a pot with the onion and water.

Bring to a boil and boil hard for 30 minutes to reduce. Strain, discarding solids (you should have about 4 cups of stock). You can prepare the gravy to this stage up to 24 hours in advance and chill the stock until needed.

When the turkey is cooked and you have transferred it to a platter to rest, tip the stock into the unwashed roasting dish and bring to a simmer on the stovetop, stirring to lift pan brownings.

Combine cornflour and lemon juice and add to turkey gravy a little at a time, stirring constantly, until gravy starts to thicken. Simmer 5 minutes.

Adjust seasoning to taste and thin with vegetable cooking water if desired. Serve hot with turkey.