Want a super-moist Thanksgiving turkey? The mayonnaise, mixed with citrus zest and herbs, melts during roasting, slowly basting the turkey. The white meat stays super moist, and the skin browns beautifully. The only thing left to do? Gobble, gobble!
For the turkey, arrange a rack in the lower third of the oven; preheat to 450°. Break off 3 cloves of garlic from the head; peel and thinly slice. Cut the remaining head of garlic in half crosswise. On a cutting board, use a large knife to chop together the sliced garlic and the orange and lemon zests. Add 1/4 tsp. salt and, using the flat side of the knife blade, flatten, mash, and scrape the mixture on the cutting board to form a paste. In a small bowl, stir the garlic paste, mayonnaise, chopped herbs, and 1/4 tsp. pepper.
Using paper towels, pat the turkey dry. Place, breast-side up, on a rack set inside a large roasting pan. Stuff the cavity with the halved head of garlic, the herb sprigs, 1 orange half,and 1 lemon half (reserving the remaining halves for the jus). Starting at the neck end of turkey, use your fingers to carefully loosen the skin from the breast and thigh meat. Using your hands, spread about a third of the mayonnaise mixture under the skin; spread the rest all over the outside of the bird. Using kitchen twine, loosely tie the legs together. Place the reserved giblets and neck in the pan.
Place the turkey in the oven; reduce the temperature to 350°. Roast for 1 hour. Rotate the pan. Continue roasting until an instant-read thermometer inserted into the thickest part of the thigh registers 165°, about 1 1/2 hours, tenting with foil if the skin is browning too quickly. Transfer the turkey to a cutting board; let rest for at least 30 minutes. Pour the drippings into a bowl. Reserve the neck; discard the giblets.
For the jus, juice the remaining orange half to yield 2 tbsp. Juice the remaining lemon half to yield 1 1/2 tbsp. Skim off the fat from the drippings and transfer 1/4 cup of the fat to another bowl; whisk the cornstarch into the fat.
Set the roasting pan across 2 burners on the stove; add the onion, bay leaves, and thyme. Cook over medium heat, stirring frequently, until the onion softens, 5 to 7 minutes. Add the reserved drippings, the stock, and the turkey neck. Increase heat to high and cook, scraping up the browned bits from the bottom of the pan, until the mixture begins to simmer, 2 to 3 minutes. Whisk in the cornstarch mixture. Reduce heat to low; simmer until the liquid is reduced by about one quarter, about 5 minutes. Remove the neck; strain the jus through a fine-mesh sieve into a pitcher. Stir in the reserved citrus juices and the parsley; season. Carve the turkey; serve with the jus.