To make the brisket, mince garlic, and combine 2 cloves worth of garlic with salt, pepper, cayenne, molasses, and mustard into a small bowl and stir to form a paste. Rub all over the brisket, and allow the brisket to rest on a plate, unrefrigerated for 1 hour.
Cut onion into slivers.
Preheat oven to 250 degrees F.
In a oven proof pot, or Dutch oven, head oil over medium high heat, add in the onions, and cook until they begin to just brown at the tips.
Add in the remaining garlic, and beef broth and scrape the bottom of the pan of any stuck bits.
Add in the brisket, fat side up, and spoon onions on top of the brisket. Cover the pot, and place in the oven for 5-6 hours, until the brisket falls apart with a fork.
To serve, uncover the pot, and allow the brisket to rest for 20-30 minutes before serving.
Stir to combine all ingredients.
Add salt to taste. Refrigerate until needed.
