Unwrap the ham and let it come to room temperature for about 1 hour before baking.
Meanwhile, position a rack in the lower middle of the oven. Preheat the oven to 300°F. Place the ham, flat-side down, in a roasting pan or other shallow baking pan. Pour about ¼-inch of water into the pan. Cover the pan tightly with foil. Roast the ham for 12 to 15 minutes per pound, or until an instant read thermometer inserted into the thickest part of the meat registers 130°F.
While the ham is in the oven, prepare the glaze. In a small bowl, whisk together the brown sugar, molasses, vinegar, and yellow mustard.
Once the ham is heated through, remove the pan from the oven and take off the cover. Increase the oven temperature to 425°F. Brush or spoon some of the glaze over the ham. Return the pan to the oven, uncovered, and roast until the glaze sizzles and begins to form a crust, about 10 to 15 more minutes. The final internal temperature of the ham should be 140°F.
Let the ham rest for at least 15 to 20 minutes before serving. Baste frequently with the pan juices. Serve warm.