Preheat the oven to 450 degrees F. Make the glaze: Whisk the pomegranate molasses, horseradish, mustard, 1/2 teaspoon salt and 1 1/2 teaspoons pepper in a medium bowl. Let it sit at room temperature to allow the flavors to meld.
Meanwhile, prepare the turkey: Remove the neck and giblets, then rinse the bird inside and out with cold water and pat dry. Rub the entire surface of the bird with the butter and season well (including inside the cavity) with salt and pepper. Truss the turkey with kitchen twine and place it breast-side up on a rack in a large roasting pan. Roast until slightly golden brown, about 45 minutes.
Reduce the oven temperature to 350 degrees F and continue roasting 1 more hour, basting with the chicken stock every 15 minutes. Brush the entire turkey with 1 cup of the pomegranate glaze and continue roasting until an instant-read thermometer inserted into the thigh registers 160 degrees F, about 15 more minutes.
Remove the turkey from the oven and brush with the remaining glaze. Tent loosely with foil and let sit 15 minutes before carving.