Place corned beef brisket in a large stockpot, cover with water, and bring to a boil. When it comes up to a boil, remove the scum that rises to the top.
To the stockpot, add the seasoning packet (that was enclosed in the corned beef brisket packaging), the peppercorns, the bay leaves, and the onions.
Turn the heat to medium low. Cover stockpot, and allow corned beef brisket to simmer for approximately 3 hours.
After 3 hours, remove the lid from the stockpot, and add potatoes, carrots, and cabbage. Place the lid back onto the stockpot, and allow the corned beef brisket and vegetables to simmer for an additional hour.
After a total of 4 hours, remove the corned beef brisket from the stockpot and transfer it to a cutting board before slicing the corned beef. Transfer the slices to a platter, and surround your corned beef with onions, potatoes, carrots, and cabbages. Serve and enjoy!