01 - Pat the beef roast completely dry with paper towels. Season all sides generously with kosher salt and freshly ground black pepper, pressing the seasoning into the meat.
02 - Heat a large skillet over medium-high heat until hot. Sear the roast on all sides until deeply browned, approximately 2 to 3 minutes per side. Transfer the seared roast to the slow cooker.
03 - Arrange the carrots, parsnips, potatoes, onions, celery, and garlic around and underneath the roast in the slow cooker insert.
04 - Whisk together the beef broth, red wine, tomato paste, and Worcestershire sauce in a bowl until the tomato paste is fully dissolved.
05 - Pour the broth mixture evenly over the roast and vegetables. Add the dried thyme and bay leaves to the liquid.
06 - Cover the slow cooker with its lid. Cook on low heat for 8 hours until the beef is fork-tender and vegetables are completely cooked through.
07 - Transfer the roast and vegetables to a serving platter and cover to keep warm. Strain the cooking liquid into a saucepan. Whisk together cornstarch and cold water until smooth, then stir into the hot liquid. Simmer over medium heat, stirring constantly, until thickened, 2 to 3 minutes.
08 - Slice the beef roast against the grain. Serve with the vegetables and spoon the thickened gravy over the top. Garnish with freshly chopped parsley.