Preheat the oven to 210 degrees. Wash the turkey thoroughly, pat it dry and rub it with salt, pepper and butter all over and inside. Finally, cover the whole turkey with bacon slices. Place it breast side down in a buttered baking dish. Arrange the celery stalks, leeks cut lengthwise, carrots, onion halves, garlic cloves and thyme sprigs in the roasting tin around the turkey.
Pour water over it and cover the whole roasting tin with foil and seal the edges. Place the roasting tin in the oven and bake at 210 degrees for 45 minutes. After this time, reduce the oven temperature to 170 degrees and bake for a further 2 hours. Then remove the foil, pull the bacon slices off the turkey and flip it onto its back. Increase the oven temperature to 200 degrees and cook the turkey until golden brown. The important thing is to let the roasted turkey come out of the oven at room temperature for another 45 minutes. Only then can you start carving it.