1) Adjust an oven rack to the lowest position and remove the other racks. Preheat the oven to 170C/Gas 3.
2) Remove the turkey parts from the breast cavities and neck, and reserve for other uses, if desired. Dry the bird well with kitchen towel, inside and out. Salt and pepper the inside of the breast cavity and stuff the onion, garlic, herbs and bay leaves inside. Set the bird on the roasting rack in a roasting tray, breast side up. Brush generously with half of the butter and season with salt and pepper. Tent the bird with foil.
3) Roast the turkey for 2 hours. Remove the foil and baste with the remaining butter. Increase the oven temperature to 220C/Gas 7 and continue to roast until an instant-read thermometer registers 74C in the thigh of the bird, about 45 minutes more.
4) Remove the turkey from the oven and set aside to rest for 15 minutes before carving.
Carve the bird and serve with the dressing.
Sage, sausage and apple dressing:
1) Preheat the oven to 170C/Gas 3.
2) Put the stuffing cubes in a large bowl and set aside. Butter a 3-litre casserole dish.
3) Melt 30g of butter in a large skillet over medium-high heat. Add the sausage and break it up with a wooden spoon. Cook until it loses most of its pink colour, but not so much that it's dry, about 5 minutes. Add the sausage and pan drippings to the stuffing cubes.
4) Melt the remaining butter in the pan. Add the onion, apple, celery and salt. Cook until the vegetables get soft, about 5 minutes. Add the stock and parsley and bring to a boil.
5) Pour the vegetable mixture over the stuffing cubes and toss until evenly moistened. Mix in the walnuts and eggs. Loosely pack the dressing in the prepared pan and cook, uncovered, until the top forms a crust, about 40 minutes.
6) Drizzle about 2 tablespoons of the turkey pan drippings or melted butter over the top. Cook until the top is crisp and golden, about 20 minutes more.
Serve immediately or warm.
Tip: Put the dressing in the oven during the last hour of cooking the turkey.
Note: To toast the walnuts, spread them out on a baking sheet and bake in a 180C/Gas 4 oven until golden, about 7 minutes.