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Heat oven to 180C/160C fan/gas 4. Pat the turkey dry all over with kitchen paper. Now weigh the bird and calculate the cooking time at 40 mins per kg.
Mix the butter with the zests and herbs, and season well. Gently push your fingers under the turkey’s skin, starting from the neck end of the bird, easing the skin away from the flesh over both the breasts and the tops of the drumsticks. Spread the butter under the skin, pushing it in as far as you can, then smooth the skin back into place. Halve the lemon and orange and put in the turkey cavity along with 2 onion halves and the bay leaves.
Rub turkey all over with oil and sprinkle with salt. Arrange remaining onions in the bottom of a large roasting tin to act as a trivet for the bird. Sit the turkey on top and completely cover both bird and tin with foil to make a tent. Roast according to your calculated cooking times.
Thirty mins before the end of the cooking time, take out the turkey and turn up the oven to 200C/180C fan/gas 6. Remove the foil and return to the oven for a further 30 mins until golden.
To test whether your turkey is cooked, pierce the fattest part of the thigh with a skewer – the juices that run out should be clear and not pink. Keep cooking if the juices are pink and re-check at 10-min intervals. If you like your turkey meat almost falling off the bone, then wiggle the leg – it should give slightly if the meat is ready. Keep cooking and basting until it does.
Rest the turkey, covered loosely with foil, for at least 30 mins and up to 1 hr before serving. Strain and reserve the cooking juices and keep the onions in the tin for making gravy.