Prep brisket: Pat brisket dry and season with salt, pepper, smoked paprika, and chili powder.
Prepare slow cooker: Place sliced onions and garlic at the bottom of the slow cooker.
Mix sauce: In a bowl, combine BBQ sauce, beef broth, Worcestershire sauce, apple cider vinegar, and brown sugar.
Add brisket: Place brisket on top of onions and garlic. Pour BBQ sauce mixture over the brisket, ensuring it is well coated.
Slow cook: Cover and cook on low for 8–10 hours or high for 5–6 hours, until brisket is tender and easily shredded.
Optional finish: For a caramelized crust, remove brisket from slow cooker and broil in the oven at 400°F (200°C) for 5–10 minutes, brushing with extra BBQ sauce.
Serve: Slice or shred brisket and spoon sauce over the top. Garnish with parsley or green onions.