01 - Rub the beef brisket generously with kosher salt and black pepper on all sides, ensuring even coverage.
02 - Heat a large skillet over medium-high heat. Sear the seasoned brisket for 3 to 4 minutes on each side until a deep brown crust forms. Transfer the seared meat to the slow cooker.
03 - Layer the sliced onions, minced garlic, carrot pieces, and chopped celery underneath and around the brisket in the slow cooker insert.
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 - Carefully remove the cooked brisket from the slow cooker and let it rest on a cutting board for 10 minutes. Slice the meat thinly against the grain for maximum tenderness.
07 - Skim any excess fat from the cooking liquid if desired. Arrange the sliced brisket and vegetables on serving plates and spoon the warm sauce generously over the top.