Preheat oven to 325°F (165°C)
Rinse brisket (optional) and pat dry
Place brisket in a Dutch oven or heavy pot, fat side up
Add spice packet from the corned beef package (if included)
Add enough water or beef broth to cover the brisket by about 1 inch
Add optional aromatics: sliced onion, garlic, bay leaf, peppercorns
Bring liquid to a simmer on the stovetop
Cover and transfer to the oven
Cook until fork-tender: about 3 to 4 hours for a 3–4 lb brisket, or until internal temperature reaches 190–205°F (88–96°C)
Add vegetables during the last 60–90 minutes: carrots and cabbage wedges; add potatoes in the last 30–45 minutes
Remove brisket and let rest 10–20 minutes
Slice against the grain
Serve with vegetables and spoon over cooking liquid