01 - Rub kosher salt and black pepper evenly over all surfaces of the trimmed beef brisket.
02 - Heat a large skillet over medium-high heat. Sear brisket for 3-4 minutes per side until deeply browned. Transfer seared meat to the slow cooker.
03 - Arrange sliced onions, minced garlic, carrot chunks, and celery pieces in the bottom of the slow cooker, placing some under and around the brisket.
04 - Whisk together beef broth, ketchup, Worcestershire sauce, brown sugar, apple cider vinegar, smoked paprika, and dried thyme until sugar dissolves completely.
05 - Pour the sauce mixture evenly over the brisket and vegetables. Cover and cook on LOW setting for 8 hours until meat is fork-tender.
06 - Carefully remove the cooked brisket and let rest on a cutting board for 10 minutes. Slice across the grain into thin strips.
07 - Skim excess fat from the cooking liquid if desired. Serve sliced brisket alongside vegetables with sauce spooned generously over the top.