Mix softened butter with salt and pepper then season the turkey cavity.
Rub the butter mixture over the turkey and cover the breast with double-layered greaseproof paper.
Refrigerate the bird overnight.
Preheat the oven to 220°C.
Bring the turkey to room temperature while the oven heats.
Place onions in a roasting tray and set the turkey on a rack above them.
Pour 235ml boiling water into the cavity and 470ml into the tray, then seal the tray tightly with foil.
Roast for 20 minutes then reduce the heat to 200°C.
Remove the foil and greaseproof paper after 1.5 hours of total cooking.
Roast for another 40 minutes without opening the oven door.
Check readiness by ensuring juices run clear when the thigh is pierced.
Rest the turkey in a warm place for 15 minutes before carving.
Strain the pan juices into a jug.
Skim off the fat and use the remaining liquid for gravy.