01 - Rub the beef chuck roast evenly with kosher salt and freshly ground black pepper on all sides.
02 - Heat a large skillet over medium-high heat. Sear the roast on all sides until deeply browned, approximately 3-4 minutes per side. Transfer the seared beef to the slow cooker.
03 - Place carrots, parsnips, potatoes, onion, celery, and garlic cloves around and beneath the beef in the slow cooker insert.
04 - Whisk together beef broth, red wine (if using), tomato paste, Worcestershire sauce, and Dijon mustard in a bowl until smooth. Pour the mixture evenly over the beef and vegetables.
05 - Tuck rosemary sprigs, thyme sprigs, and bay leaves into the liquid around the meat.
06 - Cover and cook on LOW setting for 8–9 hours (or HIGH for 4–5 hours) until the beef is fork-tender and vegetables are completely soft.
07 - Transfer the beef and vegetables to a serving platter. Discard the bay leaves and herb stems.
08 - Skim excess fat from the cooking liquid. Whisk cornstarch with cold water in a small bowl until smooth, then stir into the slow cooker. Cook on HIGH for 10–15 minutes until the sauce thickens.
09 - Slice or shred the beef against the grain. Arrange on plates with vegetables and ladle the thickened sauce generously over the top.