Heat the oven to 180C/fan 160C/gas 4. Weigh the turkey (you might need to use your bathroom scales), so you can calculate the cooking time, allowing 40 minutes per kg for the first 4kg, then 45 minutes for each kg over that weight. Push the clementines into the bird’s cavity, and rub the soft butter all over the outside, then season generously. Arrange the onion slices and thyme in the bottom of a large roasting tin. Sit the turkey on top and cover with a tent of foil so it doesn’t touch the top of the bird, but crimps tightly to the edges of the tin. Roast following to your calculation.