Step 1In a large pot, bring 2 quarts water to a boil with 2 cups kosher salt, the sugar, sliced onion, 1 head garlic, 1 bunch thyme, the celery seed and 1 tablespoon black pepper. Boil for 15 minutes. Add 1 1/2 gallons cold water. Prick the skin of the turkey in a few places on the breast and thigh and place the bird in the brine. Refrigerate, covered, in the brine for 24 hours, turning the bird a few times.
Step 2Heat the oven to 375 degrees. Remove the turkey from the brine and rinse with cold water inside and out. Dry the interior and exterior well using paper towels.
Step 3Roughly chop 1 parsnip, the carrot and the onion. Cut the head off the garlic crosswise. Season the turkey cavity with 1 teaspoon salt and one-half teaspoon pepper. Place the parsnip, carrot, onion, garlic, 1 bunch thyme and 4 tablespoons of cut-up butter into the turkey cavity. Secure the legs and wings with butcher twine.
Step 4Season the exterior of the turkey with the remaining salt and pepper and dot with the remaining butter. Place into a heavy roasting pan and roast for 2 to 2 1/2 hours, basting every 15 minutes. The internal temperature when a meat thermometer is inserted into the deepest part of the thigh, not touching the bone, should be 165 degrees.
Step 5About 30 minutes before the turkey is done, add the remaining parsnips -- sliced into quarters lengthwise -- and the pears. Continue basting.
Step 6Remove the turkey and let rest for 15 minutes before slicing. Remove the pears and parsnips and keep warm.
Step 7Remove any burned bits from the bottom of the roasting pan, if necessary. Add about one-half cup chicken broth and heat, stirring up any browned bits in the bottom of the roasting pan. Strain the juices through a fine mesh strainer, if desired.
Step 8Carve the turkey and arrange on a platter, then cut the pears into quarters or halves and arrange them around the turkey with the parsnips. Spoon jus over all.