Prepare and dry brine the turkey. Begin 24 hours before cooking. Remove the giblets and any other turkey parts from the cavity, reserving them for stock if desired.Pat the turkey thoroughly dry with clean paper towels.
Spatchcock the turkey. Using sturdy kitchen or poultry shears, cut along both sides of the backbone and remove it. Place the turkey breast-side up on a large, clean baking tray. Place both hands over the breastbone and press down firmly until you hear it crack and the turkey lies flat.
Dry brine. Sprinkle the kosher salt evenly over the entire turkey, front and back.Refrigerate uncovered for 24 hours.
Prepare for roasting. The next day, remove the turkey from the refrigerator1 hour before cooking. Pat it dry again with clean paper towels. Preheat the oven to 375 degrees F.
Make the seasoning blend. In a small bowl, combine the chicken seasoning, black pepper, onion powder, garlic powder, and paprika.
Season the turkey. Start with the back of the turkey. Drizzle with half of the olive oil, then sprinkle half of the seasoning blend evenly over the surface and rub it in. Flip the turkey breast-side up. Drizzle with the remaining olive oil and season with the remaining seasoning blend. Rub the seasoning into the turkey well, gently working some underneath the skin.
Begin roasting. Place the turkey into the preheated oven and roastuncovered for 1 hour.
Make the au poivre sauce. Melt the butter in a medium saucepan over medium-low heat. Add the shallots and garlic and sauté for about 1 minute, until fragrant. Add the freshly cracked peppercorns, rosemary, and thyme and stir well.
Add the sherry. As the butter begins to absorb into the aromatics, pour in the sherry cooking wine while stirring. Increase the heat to medium and cook until the wine has mostly reduced.
Add the cream. Pour in the heavy cream. Cover and simmer for about 5 minutes, or until the sauce begins to thicken.
Sauce the turkey. After the turkey has roasted for 1 hour, carefully remove it from the oven. Stir the pan drippings into the au poivre sauce. (Optionally, season the sauce with salt to taste, but the pan drippings should provide enough flavor.) Pour half of the au poivre sauce evenly over the turkey. Reserve the remaining sauce.
Finish roasting. Return the turkey to the oven and continue roasting for approximately 45 minutes to 1 hour, basting with the sauce and pan juices every 20 minutes. Begin checking the internal temperature toward the lower end of the cooking range. The turkey is done when an instant-read thermometer registers at least 165 degrees F in the thickest part of the breast and innermost portions of the thigh and wing.
Reduce the remaining sauce. While the turkey finishes cooking, bring the reserved au poivre sauce back to a rolling simmer. Cook, stirring frequently, until it reduces to your desired gravy consistency.
Rest the turkey. Remove the turkey from the oven and allow it to rest for 20 to 30 minutes before carving.
Serve. Spoon some of the reduced au poivre sauce over the finished turkey and serve the remaining sauce alongside it.