First make the gravy – you can do this several days ahead. Heat the oil in a non- stick pan and fry the onion and carrot with the bay leaf for 10 mins, stirring frequently, until softened and golden. Meanwhile, blend the flour with 300ml cold water to make a smooth paste.
Remove the bay leaf from the pan and pour in the stock, closely followed by the flour mixture, stirring all the time until thickened. Leave to bubble for 5-10 mins, stirring frequently, then blitz with a hand blender until completely smooth. Pour into a bowl and cover the surface of the gravy with cling film or baking parchment to prevent a skin forming as it cools. Chill.
Weigh the turkey (you may need to use bathroom scales) and calculate the cooking time by allowing 40 mins per kg for a 4.5-6.5kg turkey, then 35 mins for every kg of turkey over that weight. As a guide, a turkey this size should take 3 hrs 20 mins-3 hrs 40 mins, plus 30-45 mins resting time.
Pack the stuffing into the neck end of the turkey, smoothing down the skin and tucking it under the bird for a neat finish. Put the onion and lemon, with some bay leaves, inside the cavity. Spread the butter over the skin that covers the breasts and lay the streaky bacon on top, to cover and protect each breast and seal in the butter. Can be done the night before, but take out of the fridge 1 hr before roasting so that it comes back to room temperature.
On the day, heat oven to 180C/160C fan/ gas 4. Lay the turkey on a large sheet of double-thickness foil, long enough to completely cover it, then lift into a roasting tin and season well. Pour the wine inside the cavity, then bring the foil up over the top of the turkey and seal it well. Roast for the calculated cooking time. For the final 40 mins, open the foil and remove the bacon (serve alongside the turkey or save it for a turkey sandwich). Pour off all of the cooking juices and use a little of the buttery oil that rises to the top to baste the bird. Roast until the turkey is golden and the juices that run out of the thigh are clear, with no sign of pink, when pierced with a skewer. (If you are using a digital thermometer, the thigh should be cooked to 80C and the breast to 75C.) Leave to rest on a warm platter, covered with foil and tea towels, for up to 45 mins.
While the turkey is roasting for the final 40 mins, skim the fat from the pan juices and pour into a jug. The amount of juices you get is dependent on the type of turkey that you are cooking, as speciality bronze turkeys will give you less juice than a white turkey – so make up to 600ml with chicken stock if you need to. Tip the gravy base into a pan, gently heat it with a little of the turkey juices to loosen the consistency, then gradually blend in the remainder and cook until bubbling. Season.
To serve, carve the turkey in the kitchen and arrange on a hot platter with a little of the hot gravy poured over, or bring the whole bird to the table. Any leftover meat can be frozen in the gravy.