For the turkey: In a small bowl, mix the kosher salt, truffle salt, chopped thyme and pepper flakes. Lay the turkey flat on a rimmed baking sheet, skin-side down, and then pat it dry with paper towels. Generously sprinkle the inside of the turkey with half of the seasoning mixture, being sure the hit the thick parts a bit heavier than the thin parts. Flip the turkey and sprinkle the skin side with the remaining seasoning. Cover the turkey with a piece of loose parchment paper and refrigerate for at least 12 hours or up to 24 hours.
Preheat the oven to 450 degrees F.
Rest the turkey at room temperature for 30 minutes before roasting.
On a clean, rimmed baking sheet, scatter the thyme sprigs, celery, carrots, rosemary and onions. Pat the turkey dry one more time and place on the bed of vegetables skin-side up. Tuck the wings under the shoulders. Mix together the truffle butter and oil in a small bowl. Pour the butter mixture over the bird and brush evenly on the skin. Roast until an instant-read thermometer reads 155 degrees F in both the thigh and the thickest part of the breast, about 1 hour and 20 minutes.
Place the turkey on a cutting board to rest for 20 minutes before carving. Strain any juices from the bottom of the baking sheet and skim off any fat that may rise to the surface. Carve the turkey.
For the gravy: In a medium saucepan set over medium-high heat, heat the chicken broth, thyme sprigs and bay leaf. Bring to a simmer, then reduce the heat to maintain a simmer. In a small bowl, whisk the cornstarch and 1/4 cup water until smooth and no lumps remain. Whisk the cornstarch slurry into the simmering broth. Continue to whisk until smooth and thick, about 5 minutes. Whisk in the reserved turkey drippings, the kosher salt and truffle salt. Remove the thyme sprigs and bay leaf and serve the gravy with the carved turkey.
Prepare the turkey the day before you want to eat.