1. Cook onions in large pot of boiling salted water 2 minutes; drain. Trim root ends, leaving base intact. Peel. (Can be made 1 day ahead. Cover and refrigerate.)
2. Preheat oven to 325 degrees F. Combine sugar, vinegar, and mustard in bowl for glaze. Transfer onions to 11x7x2-inch glass baking dish. Add 2/3 cup glaze, butter, 1 teaspoon salt, 1/4 teaspoon pepper, and 1/2 cup water; toss to coat. Cover with foil.
3. Line large roasting pan with foil. Making 1/2-inch-deep slits, score ham with diamond pattern. Place ham in pan and roast 45 minutes. Place onions in oven. Roast ham and onions 25 minutes. Uncover onions. Continue roasting onions and ham 50 minutes.
4. Baste ham with some of glaze. Continue to roast ham and onions until deep brown and glazed, brushing ham with glaze every 10 minutes, about 30 minutes longer. Transfer ham to large platter. Transfer onion mixture to bowl. Serve ham, passing onion mixture separately.

Yield: 12 servings