Thoroughly rinse the corned beef under cold running water to remove surface salt. Place it in a 6 quart casserole. Add enough water to cover by an inch, and add bay leaf, peppercorns and onion. Bring to a boil over high heat, skimming off the foam and grayish matter as it rises to the surface. Cook for 2 1/2 to 3 hours or until the corned beef is tender when pierced with a fork. To reheat storebought cornedbeef wrap corned beef in foil and heat for about half an hour in a 325 degree oven until heated through.