Pat the brisket dry with paper towels. In a small bowl, combine smoked paprika, chili powder, garlic powder, salt, and pepper. Rub the seasoning mixture evenly over the brisket.
Heat olive oil in a large skillet over medium-high heat. Sear the brisket for 2–3 minutes per side, until browned.This step is optional, but it adds deeper flavor.
Place sliced onions and minced garlic in the bottom of the slow cooker.Set the brisket on top of the onions.In a bowl, whisk together beef broth, barbecue sauce, Worcestershire sauce, apple cider vinegar, brown sugar, and Dijon mustard. Pour the sauce over the brisket.
Cover and cook on low for 8–10 hours or high for 4–5 hours, until the brisket is very tender and shreds easily with a fork.
Remove brisket from the slow cooker and place it on a cutting board. Shred with two forks or slice against the grain.Return the meat to the slow cooker and toss with the sauce.
Toast buns if desired. Pile shredded brisket onto each bun and top with extra barbecue sauce, pickles, coleslaw, or sliced red onion.Serve warm.