Heat the olive oil in a large skillet over medium-high heat. Sear the brisket on both sides until browned, about 4-5 minutes per side. Remove from skillet and set aside.
In the same skillet, add the sliced onion and garlic. Sauté for 2-3 minutes until softened.
In a slow cooker, combine the beef broth, BBQ sauce, Worcestershire sauce, brown sugar, smoked paprika, onion powder, garlic powder, salt, black pepper, and cayenne pepper. Stir well to combine.
Place the seared brisket on top of the sauce mixture in the slow cooker. Add the sautéed onions and garlic on top of the brisket.
Cover and cook on low for 8-10 hours or on high for 4-6 hours, until the brisket is tender and easily shredded with a fork.
Once cooked, remove the brisket from the slow cooker and let it rest for 10-15 minutes before slicing against the grain. Serve with extra BBQ sauce if desired.