Add the sliced onions to the bottom of a 6-quart slow cooker. Sprinkle the garlic over the onions.
Whisk together the beef broth, Worcestershire sauce, and apple cider vinegar. Pour it into the slow cooker around the onions.
Pat the brisket dry with paper towels. Mix the rub ingredients together and season the brisket on all sides, pressing the rub into the meat.
Optional: Sear the brisket in a hot skillet with a little oil for 2–3 minutes per side, until browned.
Place the brisket on top of the onions in the slow cooker. Cover and cook on LOW for 8–10 hours (3–4 lb) or LOW for 10–12 hours (4–5 lb), until fork-tender.
Transfer brisket to a cutting board and rest 20–30 minutes, loosely tented with foil.
Slice against the grain, or shred with forks.
Finish (optional): Brush brisket with BBQ sauce and broil 3–5 minutes, or reduce the cooking liquid on the stove and stir in BBQ sauce to make a pan-style sauce.