Remove ham from refrigerator and let it sit at room temperature for 1-2 hours in advance. This helps the ham heat up more evenly without drying out and is a common approach to cooking larger pieces of meat. Preheat oven to 350°F.
8-10 pounds bone-in, fully cooked spiral-cut ham
Make the brown sugar glaze by whisking together the brown sugar, orange juice, whole-grained mustard, molasses, horseradish, cayenne pepper, allspice, cinnamon, and garlic powder in a medium saucepan over medium heat. Bring to a simmer for 1-2 minutes until the brown sugar is dissolved. Set aside.
1 cup brown sugar, ¾ cup fresh orange juice, ½ cup spicy whole-grained mustard, ½ cup dark molasses, 3 Tablespoons prepared horseradish,, ½ teaspoon cayenne pepper, ½ teaspoon ground allspice, ½ teaspoon ground cinnamon, ½ teaspoon garlic powder
Brush ham with brown sugar glaze using a pastry brush. Place the ham flat side down (face down) in a large pan or roasting rack and cover completely with foil.
Bake covered for 45 minutes, then uncover and baste the ham with the juices from the pan and brush with ⅓ of the brown sugar glaze.
Cover the ham with foil again and bake another 30 minutes. Remove the foil and baste the ham with it's own juices and glaze again with another ⅓ of the brown sugar glaze.
Uncover ham and bake 20-30 minutes more, until the ham reaches an internal temperature of 140°F. Remove ham from oven and baste and brush with the remaining ⅓ of the brown sugar glaze.
Loosely cover with foil and let rest for 15-30 minutes for the juices to redistribute throughout the ham.