The night before, ensure a) the turkey is already defrosted (if you have frozen yours); b) any plastic covering has been discarded; and c) the pack of giblets have been removed from the cavity. Pat the bird dry, rub a generous amount of fine salt all over the breast, legs and cavity, then return the turkey to the fridge and leave it uncovered overnight so the skin dries out a little.
Bring the turkey out of the fridge a few hours before cooking. Heat the oven to 220°C/ Fan 200°C/ Gas 7.
Fill the base of a roasting tin into which the turkey snuggly fits with the onions, celery, and garlic bulbs. Drizzle 1-2 tbsp oil over these aromatics, tumble until glossy then place the turkey on top.
Place the lemon in the cavity of the bird, plus a couple of sprigs of rosemary. Drizzle 1-2 tbsp oil over the skin of the turkey, rubbing it so that it’s glossy all over. Place in the oven, reduce the temperature to 180°C/Fan 160°C/Gas 4 and cook for 25 mins per kilo. Try not to open the oven or add other things to it during this time.
With 30 mins to go, baste the turkey with any juices in the pan. If it’s looking pale (smaller birds might), rub with a little butter and increase the temperature to 200°C. Return to the oven. When time is up, use a temperature probe to check the biggest part of the breast is around 65°C, and the legs above 80°C. If not, return to the oven for 10-15 mins more. If so, remove from the oven and leave to rest in a warm place for 60-80 mins before carving.
Meanwhile, turn the oven right up to cook your potatoes, stuffing and sides. Make your gravy by pouring any fats and juices from the roasting tin into a smaller tin or saucepan, vigorously whisking 1 heaped tbsp plain flour into that, then gradually whisking in a few tbsp vegetable or giblet stock to that. Pour through a fine sieve if a little lumpy.
To carve the bird, remove the legs from the body and slice the meat from them. The breast will be most tender and succulent if you remove it whole, then cut into slices across its width. Transfer to a warm serving platter and douse with some hot gravy before serving.