Cut onion and potatoes into chunks.
Cook in small amount of lightly salted water until tender; drain.
Sprinkle ham with mustard, brown sugar, and vinegar.
Broil one side, turn, spoon pan juice over the top, and broil until lightly browned.
Put ham on serving platter.
Pour ham drippings over drained potatoes and onion, and toss for 1 minute over low heat.
Serve around ham.