Dissolve the kosher salt and sugar in the water in large, clean bucket or stockpot. Set the turkey breast in the brine, making sure it is submerged. Cover and refrigerate for at least 8 hours or up to overnight.
Remove the turkey breast from the brine
. Pat dry and set aside. Place the butter in a bowl; add the sage and thyme
. Season the butter well with pepper and stir to combine. Set aside.
Twenty minutes before roasting, preheat the oven to 450 degrees F.
Place the turkey on a clean work surface. Using a chef's knife, remove the remaining portion of the neck and reserve it for the stock and gravy. Remove the wishbone to make carving easier; set it aside with the neck for the gravy. With your hand, carefully release the skin on both breasts to form two pockets. Rub the seasoned butter under the released skin. If there is any extra butter, massage it on the outside of the skin.
Put the celery
, and onions in a large roasting pan. Pour 1/2 cup of the chicken stock into the pan bottom to prevent the drippings
from burning. Place the prepared turkey, skin side up, on top of the vegetables. Place the pan in the oven with the wide neck end toward the rear of the oven. Roast for 15 minutes, then rotate the pan back to front. Roast
for 15 minutes more, until skin turns golden. Decrease the oven temperature to 325 degrees F and continue to roast, rotating the pan once more about halfway through the cooking, until the internal temperature in the thickest part of the breast registers 160 degrees F to 165 degrees F, 30 to 45 minutes.
Remove the pan from the oven and transfer the turkey breast to a cutting board, preferably with a moat. Cover the turkey loosely with aluminum foil. Pour the remaining 2 cups chicken stock into a saucepan. Add the reserved neck and wishbone and bring to a boil. Decrease the heat to simmer
Place the roasting pan over medium-high heat. Add the flour to the pan drippings and stir until well combined. Strain the warmed stock over the flour-vegetable combination and bring to a boil. Decrease the heat to simmer and cook until thickened, 5 to 7 minutes. Strain
the mixture into a saucepan
(the saucepan that held the stock is fine to use), pressing on the vegetables to get every drop and all the flavor. Check and make sure the sauce is thick enough to coat a spoon
; if not, continue simmering the sauce until the correct consistency is achieved. (If it's too thick, add a little water or additional stock.)
Carve the turkey breast and plate on a warm platter. Add any juices that run into the moat to the gravy
. Taste and adjust for seasoning with salt and pepper and serve with the gravy on the side.