Cook onions in a large pot of boiling, salted water for 2 minutes.
peel root vegetables, and chop into large pieces. leave base end intact for rustic presentation. place in glass baking dish
preheat oven to 325 degrees.
combine sugar, vinegar and mustard in bowl for glaze.
transfer onions to glass baking dish, with root vegetables. add glaze, butter, 1 tsp salt, 1/4 tsp pepper, and 1/2 cup water. toss to coat. cover with foil. roast for 25 mins or till soft, for crisper vegetables remove foil at end of cooking. check seasonings and serve. serves 4.