Rinse the corned beef and place in the Dutch Oven.
Cover with water, about 10 cups.
Preheat oven to 325°F.
Bring corned beef to a boil, removing film from surface, about 30 minutes.
Replace any evaporated water: add the spice packet and bay leaves.
Cover and bake for 1 hour.
Reduce the temperature to 300°F and bake for an additional 1½ hours.
Remove one up of liquid; set aside.
Nestle the onions, carrots and potatoes UNDER the cornbed beef.
Add rosemary sprigs around the vegetables and one on top of the corned beef.
Place the cabbage on top; pour the reserved liquid over the cabbage to moisten it.
Cover and bake for 1½ hours. (See notes)
Remove everything from the Dutch oven; allow the corned beef to rest for a couple of minutes before serving.
Serve with the optional dipping sauce and enjoy.