Place the brisket in a large pot and cover with cold water. Add peppercorns and mustard seeds. Bring slowly to a boil, skim any foam from the surface, then reduce to a low simmer
Cover and cook low and slow until the brisket is completely tender and easily pierced with a fork, about 2.5 to 3 hours depending on size
Add potatoes and carrots to the pot and cook 20 minutes until just tender
Add cabbage wedges and cook a final 15 minutes until bright and slightly tender but still holding their shape
Remove brisket and rest 10 minutes before slicing thick across the grain. Serve with vegetables and ladle the hot broth over everything