Photograph: Jonathan Gregson
Remove the turkey from the fridge several hours before cooking. Preheat the oven to 180°C, fan 160°C, gas 4. In a bowl, mix the butter with some seasoning. Using your fingers, carefully lift the skin away from the breast of the turkey and smear the butter under the skin on to the breast.
Place the turkey in a roasting tray with a little oil, then put the onion, lemon and herbs in the cavity; tie the legs with string. Roast in the pre-heated oven for 20 minutes per kilo (1 hour 40 minutes for a 5kg bird).
For the stuffing balls, melt the butter in a pan and add the onion. Cover with a lid and gently cook the onion for 8-10 minutes. Cool.
Squeeze the sausagemeat from the sausages into a bowl, discarding the skins, mix with a wooden spoon to break it up. Add the cooled onions, herbs and the chestnuts. Finally, add the breadcrumbs and mix thoroughly. Using your hands, roll the stuffing into 18 balls and transfer to an oiled baking tray.
Once the turkey is cooked, remove from the oven and leave the turkey to rest for 30-45 minutes. Reserve the juices in the tin for the gravy. Cook the stuffing balls for 25-30 minutes until golden and cooked through.
Meanwhile, to make the gravy, skim off the excess fat from the juices and put the roasting tray on the hob. Slowly add the stock and bring to the boil, then simmer for 10 minutes. If you prefer a thicker gravy, add a tablespoon of cornflour that you have mixed with a little cold water, whisking it in until smooth. Season the gravy and pour it through a sieve into a serving jug.
Serve the turkey with stuffing balls and all the trimmings.
Get ahead: make and shape the stuffing balls a day ahead; chill. They can also be frozen; defrost before cooking.