Make the Schezuan dry-brined turkey: Place the coriander, fennel and Schezuan peppercorns in a small skillet over medium heat and toast until fragrant, about 5 minutes. Let the mixture cool, then transfer to a spice grinder and pulse until the spices are coarsely ground. Transfer the spice mix to a small bowl and stir in the salt; set aside.
Place the turkey on a clean work surface and trim off any excess pockets of fat or skin around the turkey. Flip the turkey so it is laying breast-side up. Using the heels of your hands, press down on the center of both breasts until you hear a cracking sound and the bird flattens.
Pat the bird very dry using a kitchen towel and rub the spice mixture on both sides. Transfer the turkey, breast-side up, to a wire-rack placed over a parchment-lined sheet tray. Tuck the wings underneath. Store the turkey in the refrigerator for at least overnight and up to 24 hours.
Remove the turkey an hour before roasting to take the chill off.
Place the reserved backbone and neck in a roasting pan with the carrots, onions, thyme, bay leaf and stock. Place the turkey on a wire rack.
Preheat the oven to 425°. Set one rack in the middle of the oven and another one below for the roasting pan. Place the turkey on the center rack of the oven. Place the roasting pan on the rack below the turkey and roast both for 30 minutes. Reduce the heat to 350° and finish roasting until the turkey is a deep golden brown and crisp or until an instant-read thermometer inserted into the thickest part of the thigh reads 165°, another 40 to 50 minutes. Remove the turkey from the oven and, using two grilling spatulas, transfer the bird to a platter. Let the turkey rest for 30 minutes before carving.
Meanwhile, make the gravy: Transfer the roasted neck-and-backbone turkey mixture along with the stock to a medium saucepan. Bring the liquid to a boil and reduce it by half, 10 minutes. Strain the stock through a fine sieve and discard the solids. Skim the stock of any excess fat before placing it back into the saucepan. Bring to a simmer.
Meanwhile, in a small saucepan over medium heat, melt the butter. When the butter begins to foam, whisk in the flour. Reduce the heat to low and whisk until the roux is golden brown, 2 to 3 minutes.`
Add 1 to 2 cups of hot stock to the roux, whisking constantly. Continue until all the stock has been added. Simmer until the gravy has thickened and the flavors have melded, about 5 minutes. Stir in the cream and season the gravy with salt and pepper. Carve the turkey and serve with the warm gravy.
