01 - Pat the beef chuck roast dry with paper towels. Season generously on all sides with kosher salt and freshly ground black pepper, pressing the seasoning into the meat.
02 - Heat a large skillet over medium-high heat until smoking hot. Sear the roast on all sides until deeply browned, approximately 3-4 minutes per side. Transfer the seared beef to the slow cooker.
03 - Arrange the carrots, parsnips, potatoes, onion, celery, and garlic cloves around and beneath the beef in the slow cooker, creating an even layer.
04 - In a medium bowl, whisk together the beef broth, red wine, tomato paste, Worcestershire sauce, and Dijon mustard until smooth and fully combined.
05 - Pour the broth mixture evenly over the beef and vegetables. Tuck the rosemary sprigs, thyme sprigs, and bay leaves into the liquid around the meat.
06 - Cover and cook on LOW setting for 8 to 9 hours, or on HIGH for 4 to 5 hours. The beef is done when it forks apart easily and vegetables are tender.
07 - Carefully transfer the beef and vegetables to a serving platter using a slotted spoon or tongs. Discard the bay leaves and woody herb stems. Tent loosely with foil to keep warm.
08 - Skim excess fat from the cooking liquid. Whisk cornstarch with cold water in a small bowl until smooth, then stir into the hot liquid. Cover and cook on HIGH for 10-15 minutes until thickened.
09 - Slice or shred the beef against the grain. Arrange meat with vegetables on plates and spoon the thickened sauce generously over the top.