Rinse the corned beef brisket under cold water
Place the brisket in a large pot or Dutch oven
Add enough water to cover the brisket
Add the spice packet if included
Add optional aromatics such as onion, garlic, bay leaves, and peppercorns
Bring the water to a boil
Reduce to a low simmer
Cover and cook until tender, about 2.5 to 4 hours depending on size
Check occasionally and add more water if needed to keep the brisket covered
Test tenderness with a fork
Remove the brisket from the pot
Let it rest for 10 to 15 minutes
Slice thinly against the grain
Serve warm