If you have time the day before, salt the turkey. Make the salt mix by grinding the salt with the thyme leaves and pepper in a spice grinder or with a pestle and mortar. Sit the turkey in a roasting tin and season all over, inside and out, using about two-thirds of the salt. Leave the turkey in the tin, breast-side up, cover with cling film and put in the fridge for up to 24 hrs.
If you don’t have time, or there is no room in the fridge, simply prepare the salt and season the turkey generously before roasting.
To make the bacon butter, gently cook the bacon in a dry frying pan for about 10 mins, stirring occasionally, until the fat has seeped into the pan and is sizzling gently, and the bacon is crisp. Take off the heat and leave to cool slightly. Scrape the bacon and the fat into a food processor with the butter and maple syrup. Blitz to combine, scraping down the sides of the processor from time to time. Roll up the butter in cling film to form a log and place in the fridge. Can be made a few days ahead or frozen for 1 month.
Remove the turkey from the fridge an hour or so before you want to cook it. Remove the bacon butter from the fridge to soften up. Heat oven to 180C/160C fan/gas 4. Calculate your cooking time based on 40 mins per kg for the first 4kg of the turkey, plus 45 mins for every kg after that.
Gently push your fingers under the skin of the turkey, starting from the neck, until you can push your whole hand in down the length of the breast. Take care not to tear the skin. Use your hands to spread the bacon butter under the skin so that it covers the entire breast area, and there is butter in the crevice between the thigh and the main body. Pop a sprig of thyme under the skin of each breast, then smooth the skin over with your buttery hands. Place the onion, lemon, bay leaves and garlic in the cavity.
Cover the tin loosely with foil and roast for the calculated cooking time – 30 mins before the end of cooking, increase the oven temperature to 200C/180C fan/gas 6, remove the foil, baste the turkey and return to the oven. When the turkey is beautifully brown and cooked through, remove from the oven and leave to rest on a warm platter covered in foil for up to 1 hr.
Pour the fat away from the roasting tin, leaving just the juices. If your tin is flameproof, pour in the Sticky Port gravy (see 'Goes well with', right) to reheat with the juices; if not, tip them both into a saucepan and reheat. Serve the remaining salt mix as a seasoning alongside the rest of the meal.