Allow the turkey to come to room temperature for 2 hours.
Preheat the oven to 425 degrees F.
Remove 2 tablespoons of ranch seasoning from 1 packet and reserve. In a small bowl, mix together 1 stick (8 tablespoons) of butter with the remaining ranch seasoning from both packets until well incorporated.
Arrange the carrots, celery and onions in the bottom of a roasting pan, then put a roasting rack over them. Put the turkey on the rack and spread the ranch butter all over and underneath the skin. Season generously with salt and pepper. Reduce the oven temperature to 350 degrees F and put the turkey inside to roast.
In the meantime, add the maple syrup and 2 cups of chicken stock to a saucepan, then stir in the 2 tablespoons reserved ranch seasoning. Bring to a simmer just to warm through and incorporate the seasoning. Roast the turkey, basting with this liquid every 20 minutes, until a thermometer inserted in the thickest part of the thigh reads 160 degrees F (allow about 1 hour of cooking time for every 3 pounds of turkey). Tent with a piece of aluminum foil and let rest at least 30 minutes. (There will be carryover cooking and the temperature will rise to 165 degrees F while resting.)
For the gravy, strain the juices left in the bottom of the roasting pan into a fat separator and pour the fat off the top. Put the roasting pan on a burner on top of the stove over medium heat. Add the remaining 2 tablespoons butter and whisk in the flour, cooking for a minute. Slowly whisk in the remaining 2 cups chicken stock along with any remaining braising liquid and the juices from the roasting pan. Season with salt and pepper and serve alongside the turkey.
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