1. Slice the onions and place in the bottom of the slow cooker.  Pour the broth over the onions.
2. Place the brisket in a large bowl and cover the top and bottom with 2/3rds of the onion mixture.  Place brisket, fat side up, in the slow cooker.
3. Spread the dijon mustard over the top of the brisket and sprinkle the remaining 1/3rd of onion mixture over the top.
4. Set slow cooker on low for 9 hours.
5. When brisket is cooked, carefully remove it from the slow cooker and place it in a large roasting pan.  If using, sprinkle brown sugar over brisket.  Set the brisket on the top 1/3rd of the oven and turn the broiler on high for about 5 minutes.  Keep a close eye on the brisket as it can easily burn.
6. Remove from the oven and set aside for at least 30 minutes before cutting.

To Make "Au Jus"

1. Allow the drippings to cool completely.  Skim the fat off the top of the drippings and discard.
2. Pour onions and remaining broth in a high-speed blender and blend on high for 1-2 minutes.  Taste and add any additional salt or pepper if needed.
3. To serve, pour over brisket or place in a small side bowl for dipping.