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 the seasoned brisket for 3 to 4 minutes per side until a deep brown crust forms. Transfer the seared meat to the slow cooker insert.
03 - Arrange the sliced onions, minced garlic, carrot pieces, and chopped celery around and beneath the brisket in the slow cooker.
04 - Whisk together the beef broth, ketchup, Worcestershire sauce, brown sugar, apple cider vinegar, smoked paprika, and dried thyme in a bowl until the sugar dissolves completely.
05 - Pour the sauce mixture evenly over the brisket and vegetables. Cover with the lid and cook on LOW setting for 8 hours until the meat offers no resistance when pierced with a fork.
06 - Transfer the cooked brisket to a cutting board and let rest for 10 minutes to allow juices to redistribute. Slice the meat against the grain into thin strips.
07 - Skim any excess fat from the surface of the cooking liquid. Serve the sliced brisket alongside the tender vegetables, spooning the reduced sauce generously over each portion.