Wash the seasonings that were in the vacuum-pack bag off the corned beef, and put it in a large pot of cold water.
Put the seasonings, the crab boil, and the onion into the pot. Bring the pot to a boil, then lower to the barest possible simmer. Simmer with the cover on for three and a half hours.
Boil the cabbage separately in about two gallons of water with 2 Tbs. salt.
When the corned beef is cooked, drain it from the water and let it stand for about 20 minutes. Slice it against the grain, noting that the grain in brisket has a way of changing directions as you slice it. The thinner you slice it, the better.