Set the turkey, breast side down, on a cutting board. Use kitchen shears to cut along both sides of the backbone, removing a strip about 2 inches wide; reserve this piece for making stock and/or soup. Turn the bird breast side up and use the heels of your hands to press down on both sides of the breast, flattening it slightly. Clean the cutting board; rinse the bird well, pat dry with paper towels and return to the cutting board.
In a small bowl, combine the salt, thyme, sage, rosemary and ground fennel seed. Use your hands to carefully loosen the skin and work the herb mixture between the breast and leg meat and the skin. Sprinkle the mixture all over the outside of the bird. Place the bird on a flat rack (or on top of whole carrots and celery). Cover loosely and refrigerate for at least 1 day and up to 2 days.
One hour before you are ready to roast the turkey, position an oven rack on the lowest level and preheat to 425 degrees. Place the bird on its rack in the roasting pan, skin side up. (If roasting with the stuffing under the bird, evenly spread the stuffing in the pan, then place the turkey on top of the stuffing skin side up; no rack is needed). You might need to bend the legs up a bit, and the drumsticks might overlap the pan's edges; that's okay. Tuck the wing tips behind the wings and massage half of the butter between the breast meat and its skin.
When ready to roast the turkey, melt the remaining butter and brush some of it over the top of the bird. Roast for 30 minutes, then reduce the temperature to 375 degrees and brush the skin with the remaining melted butter. Roast for about 1 3/4 hours or until an instant-read meat thermometer inserted into the thickest part of the thigh meat registers 170 degrees and the skin is crisp and nicely browned. Transfer the turkey to a large cutting board. Loosely cover with aluminum foil and let rest for 30 minutes before carving.
NOTE: To toast fennel seeds, heat them in a dry skillet over medium heat, shaking the pan frequently, until lightly browned and fragrant, 4 to 8 minutes.