For the turkey: 4 days before service, place the salt, sage, thyme, black peppercorns, and allspice into a spice grinder and pulse until the peppercorns and allspice are coarsely ground, 5 to 6 pulses. Set aside.
Set the turkey, breast-side down, on a large cutting board with the tail closest to you. Use an electric knife or heavy-duty kitchen shears to cut up one side of the backbone. Turn the bird around and cut back down the other side of the spine. Reserve the backbone for stock, discard any fat pockets or excess skin found inside the turkey. Turn the turkey breast-side up and use the heel of your hands to press down on both breasts, until you hear a cracking sound and the bird has flattened slightly.
Rub the seasoned salt on both sides of the turkey. Place the turkey on a parchment paper-lined half-sheet pan, breast-side up with legs running with the long side of the pan. Store, uncovered, in the refrigerator for 4 days.
After 4 days, remove the turkey from the refrigerator and leave at room temperature for 1 hour.
For the panzanella: Place the parsnips and rutabaga in a large roasting pan, toss with vegetable oil, and set aside.
Place one rack in the middle of the oven and a second one far enough below so the roasting pan will fit. Heat the oven to 425ºF. Place the turkey directly on the middle rack of the oven with the legs perpendicular to the metal bars of the rack. Place the roasting pan with the parsnips and rutabaga on the rack below the turkey and roast both for 30 minutes.
After 30 minutes, reduce the heat to 350ºF. Add the red onion to the roasting vegetables and stir to combine. Continue to roast both the vegetables and the turkey until a probe thermometer inserted into the thickest part of the breast registers 155ºF, an additional 40 to 50 minutes.
Remove the turkey from the oven onto a cooling rack set inside a half-sheet pan and rest for 30 minutes.
Add the Brussels sprouts, bread cubes, and garlic to the roasting vegetables, stir to combine, and roast for an additional 15 minutes. Remove the vegetables from the oven and immediately transfer to a serving bowl. Pour the apple cider vinegar in the warm roasting pan, stir, and scrape off any browned bits from the pan. Pour the vinegar mixture over the salad, add the thyme, and toss to combine. Season with salt and pepper as desired. Serve warm or at room temperature.
Carve the turkey with an electric knife and serve with the panzanella.