Preheat the oven to 220°C/Gas 7. Mix the softened butter with salt, pepper, olive oil, lemon zest and juice, crushed garlic, and parsley until well combined.
Remove the giblets from the turkey. Season the cavity with salt and pepper, then fill it with the onions, lemon, garlic halves, and 2 bay leaves.
Carefully loosen the skin over the breasts and legs without tearing it. Push half of the herb butter underneath the skin and gently massage it over the breasts.
Place the remaining 4 bay leaves under the breast skin. Rub the remaining butter all over the outside of the turkey, then season with salt and pepper and drizzle with olive oil.
Place the turkey breast-side up in a roasting tray and roast for 10–15 minutes.
Remove from the oven and baste with the pan juices. Lay the bacon rashers over the breast and baste again.
Reduce the oven to 180°C/Gas 4 and roast for about 2½ hours, basting occasionally.
Check the thickest part of the leg with a skewer; the juices should run clear rather than pink. If necessary, continue roasting and check again every 15 minutes.
Transfer the turkey to a warmed platter and let it rest for at least 45 minutes. Remove the bay leaves before carving, then serve with hot gravy and your favorite festive sides.