For the mustard maple glaze: Whisk together the maple syrup, horseradish, mustard and ancho in a bowl and season with salt and pepper. Cover and let sit at room temperature for at least 30 minutes to allow the flavors to meld. (Can be made up to 2 days in advance and stored, covered, in the refrigerator. Bring to room temperature before using.)
For the turkey: Remove the turkey from the refrigerator 1 hour prior to roasting. Preheat the oven to 450 degrees F.
Scatter the carrots, celery and onions in a large roasting pan and set a roasting rack on top. Place the turkey on the rack and sprinkle the cavity with salt and pepper. Brush the entire turkey with the melted butter and sprinkle liberally with salt and pepper. In a medium saucepan, combine 4 cups of the chicken stock and the 1/2 stick butter; keep warm over low heat. Ladle a cup or so over the bird.
Transfer the turkey to the oven and roast until light golden brown, 45 minutes. Reduce the heat to 350 degrees F and continue roasting, basting with a few cups of the warm chicken stock mixture every 15 minutes, until an instant-read thermometer inserted into the thigh registers 145 degrees F. Return the turkey to the oven and cook, basting with mustard maple glaze every 10 minutes, until it reaches an internal temperature of 155 degrees F. The total roasting time will be about 2 hours 45 minutes.
Transfer the turkey to a large cutting board, tent loosely with foil, and let rest for 30 minutes to 1 hour before slicing. Meanwhile, strain the drippings from the roasting pan into a bowl, discarding the solids. If needed, add enough of any remaining chicken stock to make 6 cups of liquid.
For the gravy: Melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook until light golden brown. Slowly whisk the drippings into the flour mixture. Bring to a boil, whisking until the mixture begins to thicken and the flour taste has been cooked out, about 10 minutes. Add more stock if needed and a splash of white wine if desired, and cook for 2 minutes longer. Fold in the herbs and season with salt and pepper. Serve immediately.
Have lots of extra stock on hand for the meal to keep the turkey moist on Thanksgiving day and the day after, when reheating. I like to reheat turkey by putting it in a baking dish, covering it with stock, covering dish with foil and warming it in a 350 degree oven.