Take a 4-6 quart crock-pot and add carrot chunks and chopped onions to it. Take the corned beef and wash it thoroughly, put it on paper towels and pat it to dry. Add the corned beef to the crock-pot with the onions and carrots. Sprinkle the seasoning over it. Open the beer can and pour over the brisket, followed by spreading mustard gently. Take another small bowl and mix water and brown sugar in it. Also pour this on the brisket. Cover it with the lid and let it cook on low setting for about 10-12 hours.
After it has cooked for the desired amount of time, remove it from the crock-pot and cover with foil till it is still hot. Put the brisket in warm oven with 200 degree temperature. In the same crock-pot with the remaining liquid, add the cabbage. Cover it with the lid and make the settings to high. Cook for around 30 to 40 minutes. By then the cabbage will get tender and crisp.
After the cabbage is ready, take the beef out of the oven. Cut it into thin slices across the grains. If you like the liquid, drizzle it on the beef before serving. You could also add extra mustard on one side and honey mustard can also be used. If your crock-pot is new, its heat intensity will be more. You should check the beef after 9 hours. The liquid will prevent it from burning even if you leave it for little longer.