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 deep brown crust forms. Transfer to slow cooker.
03 - Place sliced onions, minced garlic, carrot pieces, and chopped celery in the bottom of the slow cooker. Set seared brisket on top of the vegetables.
04 - Whisk together beef broth, ketchup, Worcestershire sauce, brown sugar, apple cider vinegar, smoked paprika, and dried thyme in a bowl until sugar dissolves.
05 - Pour the sauce mixture evenly over the brisket and vegetables.
06 - Cover slow cooker and cook on LOW setting for 8 hours until brisket is fork-tender and easily pulls apart.
07 - Remove brisket from slow cooker and let rest on cutting board for 10 minutes. Slice against the grain into thin strips.
08 - Skim excess fat from cooking liquid if desired. Serve sliced brisket with vegetables and spoon sauce generously over the top.