Combine salt, pepper, smoked paprika, thyme, and rosemary. Pat the chuck roast dry, rub the spice mix all over, and let it stand at room temperature for 15 minutes while you prep the vegetables.
Layer the onion, celery, and garlic in the bottom of a 6–7 quart slow cooker. Scatter the halved potatoes on top. Keep the carrots aside for later.
Heat the oil in a large skillet over medium-high heat. Sear the roast until well browned, about 3 minutes per side. Transfer the roast to the slow cooker, nestling it over the potatoes. Add the bay leaves.
Lower the skillet heat to medium. Add the tomato paste and cook, stirring, for 30 seconds. Stir in the balsamic vinegar and Worcestershire, scraping up the browned bits. Pour in the beef broth and simmer 1 minute. Pour this liquid over the roast and vegetables.
Cover and cook on LOW for 7 1/2 to 9 hours (or on HIGH for 4 1/2 to 5 hours). About 2 hours before the end of cooking, tuck the carrot pieces around the roast, re-cover, and continue cooking until the meat shreds easily and the vegetables are tender.
Transfer the roast to a board and tent with foil for 10 minutes. Discard bay leaves. Skim excess fat from the cooking liquid in the slow cooker.
For the gravy, strain 2 cups of the cooking liquid into a saucepan. Whisk cornstarch with cold water and Dijon until smooth, then whisk into the saucepan. Bring to a simmer over medium heat, whisking, until glossy and thickened, 2–3 minutes. Season to taste with additional salt and pepper if needed.
Shred the roast into large pieces and return it and the carrots to the slow cooker (or arrange on a platter). Spoon some gravy over the top and serve the rest on the side with the potatoes and vegetables.