Remove corned beef from package and rinse off brine under cool running water. Rinsing the brine off helps to remove the excess salt from the meat. Also trim any excess fat from the meat.
Place the corned beef in a large, heavy pot and cover with water. Add the spice packet included with the corned beef. Bring to a boil and then reduce heat and let simmer for about 2- 3 hours.
At about 2 hours, add chunks of onions, potatoes and carrots. Bring back to a simmer and cook until vegetables are tender. You can also add wedges of cabbage and other root vegetables such as turnips, parsnips and beets, if desired. Just make sure that you do not overcook the vegetables. If the vegetables get done before the meat, remove them from the pot and keep warm.
You can tell when the corned beef is done by piercing the meat with a thin, sharp knife or a fork. When the meat allows the knife or fork to pass into the center of the meat with little resistance, it is done. But to be sure of the doneness, use a food thermometer to test the meat. The meat should be at an internal temperature of 145 degrees.
Serve the sliced corned beef on a platter surrounded by the vegetables. The juice that is made while the meat and vegetables are cooking is very good to serve on the vegetables. Delicious!