Rinse the corned beef and place it in a large heavy pot or a Dutch oven along with the onions, garlic, cloves, bay leaves, and salt and pepper. Add cold water to cover. Bring to a boil, reduce the heat, and simmer, covered, until the beef is tender, about 4 hours.
Remove corned beef from cooking liquid, cool and then refrigerate, well wrapped, overnight.
The next day, combine the corn syrup and mustard in a small saucepan. Simmer gently, stirring occasionally, for about 8 minutes.
Position a rack in the oven 5 inches from the broiler. Preheat the broiler. Trim excess fat from the meat, place the corned beef in a broiler pan, and brush some of the mustard glaze on it. Broil in the oven for 10 minutes, while brushing more glaze on it every few minutes. Watch to make sure it doesn’t burn or overcook.