Up to two days ahead, make the salt mix by grinding the salt with the thyme leaves and pepper in a spice grinder or using a pestle and mortar. Sit the turkey in a roasting tin and gently push your fingers under the skin, 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.
Season the bird all over with the salt mix, 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 the turkey will be. For the crispest skin, leave the turkey uncovered so it dries out, but loosely cover it if you prefer.
Tip the butter, vegetable stock powder and lemon zest into a small bowl and mix until completely combined. Cover and chill. Can be made a few days ahead or frozen, wrapped in cling film, for one month.
Remove the turkey from the fridge an hour or so before you want to cook it. Remove the butter from the fridge to soften. Heat oven to 180C/160C fan/gas 4. Work out your cooking time based on 40 mins per kg for the first 4kg, plus 45 mins for every kg after that.
Use your hands to spread the butter under the skin so that it covers the entire breast area and work it into the crevice between the thigh and the main body, then smooth the skin over with your hands. Put the onion, lemon halves, and garlic in the cavity.
Cover the tin loosely with foil and roast for the calculated cooking time. Around 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 with foil for up to 1 hr. Save the juices from the tin to make gravy.