In a medium bowl, whisk together the BBQ sauce and beef broth. Set aside.
In a small bowl, combine the dry rub ingredients.
Pat the brisket dry with paper towels. Then massage the dry rub all over the brisket.
Heat a large skillet over medium-high heat. Once hot, add oil and then sear the brisket on all sides until browned, about 3-4 minutes each side.
Transfer the brisket into the bottom of the slow cooker pot and surround with the sliced onions.
Pour the BBQ sauce and broth mixture over the brisket.
Cover with the lid. Cook on LOW for 8-10 hours. For a smaller 2 pound brisket, cook for less time on LOW for 6-8 hours.
Once cooked, remove the brisket from the slow cooker and transfer to a cutting board. Slice or shred the meat then return it back to the slow cooker and stir into the sauce.
Garnish with fresh parsley if desired.