Arrange a rack in the lower third of the oven and remove upper racks. Preheat oven to 325ºF.
Remove ham from packaging and set on a rack in a large roasting pan. Trim off any skin and score fat in a crosshatch pattern, cutting about ¼-inch deep. Pour 1 cup water in the pan and roast, uncovered, for 1 hour. Add more water to pan, as needed, to prevent drippings from burning.
Meanwhile, melt butter in a small saucepan over medium heat. Add brown sugar, balsamic, Dijon mustard, rosemary, ½ teaspoon Kosher salt and black pepper. Bring to a low simmer and cook for 3-5 minutes, stirring frequently, until mixture resembles a thin syrup. Remove from heat and set aside.
Brush ham with a third of the balsamic glaze and roast 20 minutes. Baste two more times, every 20 minutes, tenting with foil if browning too quickly. Ham is ready when crust is golden brown and an instant-read thermometer inserted into the thickest part of the meat registers 140°F.
Transfer to a cutting board and let rest 15 minutes before slicing. Serve warm.