Cook onions in large pot of boiling salted water 2 minutes; remove from heat and drain. Trim root ends, leaving base intact. Peel; set aside. Onions may be made 1 day ahead. Covered and refrigerate.
Preheat oven to 325 degrees F. Line a large stainless-steel Roasting Pan with aluminum foil.
In a small bowl, combine brown sugar, balsamic vinegar, and Dijon mustard to make the balsamic glaze.
Place prepared onions in a 11x7x2-inch glass baking dish. Add 2/3 cup of the prepared balsamic glaze, butter, salt, pepper, and water; toss to coat. Cover with aluminum foil.
Making 1/2-inch-deep slits, score ham with diamond pattern. Place ham in prepared roasting pan and bake 45 minutes. Place the baking dish with the onions in the oven. Roast ham and onions 25 minutes. Uncover onions, and continue roasting onions and ham an additional 50 minutes. Baste ham with some of balsamic glaze. Continue to roast ham and onions until deep brown and glazed, brushing ham with balsamic glaze every 10 minutes, about 30 minutes longer or until internal temperature reads 140 degrees F. on a meat thermometer. Remove from oven and transfer ham to large platter.
Transfer onion mixture to bowl. Serve ham, passing onion mixture separately.
Makes 12 servings.
* A spiral ham may be substituted, but cooking times will be shorter. Also no need to cut slits.
Balsamic and Dijon Glazed Ham Recipe: https://whatscookingamerica.net/pork/balsamicglazedham.htm