Up to two days ahead, season the bird all over with flaky sea salt – inside and out and under the skin. Leave the turkey in the tin, breast-side up, and put in the fridge for up to two days; the longer you salt it, the more succulent it will be. Weigh the turkey and calculate the cooking time by allowing 40 mins per kg for the first 4kg, then 45 mins for every additional kg. As a guide, a turkey of this size should take 3½-4 hrs plus 30-45 mins resting.
To make the butter, mash the garlic with the thyme, lemon zest and a pinch of salt using a pestle and mortar, then beat in the butter until well mixed. Can be prepared up to two days ahead and chilled.
Gently push your fingers under the skin of the turkey, starting from the neck, until you can push your whole hand down the length of the breast – take care not to tear the skin. Spread the butter under the skin so that it covers the breast meat. Lay the pancetta (or bacon) on your work surface in two rectangles of six overlapping slices. Carefully push each rectangle under the skin to cover and protect each breast. This can be done the night before; take the turkey out of the fridge 1 hr before roasting to bring it back to room temperature.
On the day, heat oven to 180C/160C fan/gas 4. Tip the whole shallots into a bowl with the thyme, bay and garlic butter, then season and toss to coat. Lift the turkey into a roasting tin, massage the olive oil into the skin and season well if you haven’t already seasoned ahead. Tip the shallots into the roasting tin, around the turkey, and stuff the lemon halves into the cavity. Cover the tin loosely with foil and roast for the calculated cooking time.
For the final 30 mins, remove the foil and pour off all of the cooking juices (reserve them for the gravy). Spoon the shallots into the cavity and increase the oven to 200C/180C fan/gas 6. Roast for the final 30 mins until the turkey is golden and the thigh juices run clear when pierced with a skewer, or until a digital cooking thermometer reads over 70C. Leave the turkey to rest on a warm platter covered with foil – it will stay warm for about 1 hr. If you want to make turkey gravy, pour the fat off the juices and add the gravy to the roasting tin. Bring to the boil, then pour into a gravy jug. To serve, bring the whole turkey to the table, along with the herbs, if using. Carve onto a hot platter with a little of the hot gravy poured over. Any leftover meat can be frozen in the gravy, or used in leftovers.