Brisket is a long, somewhat flat cut of meet.You want a thin layer of fat on one side of the beef, but trim any extra pieces of fat from the beef.
You can either combine all the dry rub ingredients and then coat the meat with the rub, or season with each ingredient individually. Coat the meat well on both sides and edges.
Heat up a large frying pan (on medium heat) and then brown the brisket briefly for a minute or two on each side and a bit on the edges.
Place the diced onions on the bottom of a crock-pot or Dutch oven, and then place the browned brisket over the onions (fat side facing up).
Cover and cook for about 4 hours, or until the meat is tender (inserting a fork to test). Use a high setting on your crock-pot for about 4 hours, or on a low setting for about 8 hours. This will vary somewhat by oven or crock pot.
Remove the brisket, cool and slice across the grain.
Use the gravy and onions at the bottom of the pot to top off the brisket slices.