Make the Gravy (Can Be Done Ahead): Heat the olive oil in a non-stick pan and fry the chopped onion, carrot, and bay leaf for 10 minutes, stirring frequently until softened and golden.
While it cooks, mix the flour with 300ml cold water to make a smooth paste.
Remove the bay leaf, add the chicken stock, and then the flour mixture, stirring continuously until thickened.
Let it simmer for 5-10 minutes, stirring frequently.
Blend with a hand blender until smooth.
Pour into a bowl, cover with cling film to prevent a skin, and chill.
Prepare the Turkey: Weigh the turkey to calculate cooking time: allow 40 minutes per kg for a 4.5-6.5kg turkey, then 35 minutes per kg over that weight.
This size turkey should take around 3 hours 20 minutes to 3 hours 40 minutes, plus 30-45 minutes resting.
Stuff & Season: Pack the stuffing into the neck end of the turkey, smooth the skin, and tuck it under for a neat finish.
Place the onion, lemon halves, and bay leaves in the cavity.
Spread the butter over the breast skin and layer the streaky bacon on top to protect and seal in the butter.
(This can be done the night before; bring to room temperature 1 hour before roasting).
Roast the Turkey: Preheat the oven to 180°C (160°C fan) or gas mark 4.
Place the turkey on a large sheet of foil and transfer to a roasting tin.
Pour the wine into the cavity, season, then wrap the foil over the turkey and seal tightly.
Roast for the calculated time.
Finish Roasting: For the last 40 minutes, open the foil and remove the bacon (serve with the turkey or save for leftovers).
Pour off the cooking juices and baste the turkey with a little of the buttery oil that rises to the top.
Roast until golden and the thigh juices run clear.
(If using a thermometer, cook the thigh to 80°C and the breast to 75°C).
Let rest on a warm platter, covered with foil and tea towels, for up to 45 minutes.
Finish the Gravy: While the turkey rests, skim the fat from the pan juices and add to a jug.
Adjust with chicken stock to reach 600ml if needed.
Gently heat the gravy base, gradually add the pan juices, and cook until bubbling.
Season to taste.
Serve: Carve the turkey and arrange on a hot platter with some of the hot gravy poured over, or bring the whole bird to the table.
Leftover turkey can be frozen in the gravy for later use.