Be forewarned: In cooking, the meat shrinks by half.

1.   Stud onions with cloves. Rinse corned beef in cold water to remove brine. Put beef in a large pot and add onions, carrots, bay leaves, peppercorns, and enough water to cover. Bring to a boil over high heat, cover, and reduce heat to low. Simmer for 2 hours, skimming occasionally.
2.   Wash cabbage, remove core and any torn leaves, then cut into 6 wedges. Add cabbage and potatoes to beef, then cover and simmer until potatoes are tender, about 25 minutes.
3.   Transfer beef to a cutting board and cover with a plate weighted with heavy cans (weighting makes meat easier to slice). Transfer onions, carrots, cabbage, and potatoes to a platter. Remove cloves from onions. Strain cooking liquid, discarding bay leaves and peppercorns. Return liquid to pot and cook over high heat until reduced by one-third, 20-30 minutes. Season to taste with salt and pepper.
4.   Return vegetables to stock and heat through for about 5 minutes. Remove plate and cans from meat and cut across the grain, in 1/4"-thick slices. Arrange beef and vegetables on warmed platter. Moisten with stock. Serve with additional stock and hot mustard if you like.