Pat the roast dry and season all over with your favorite beef seasoning. For extra flavor, sear all sides in butter or olive oil before adding to the crockpot (optional).
Place the onion, garlic, carrots, and celery underneath the chuck roast, then arrange the potatoes around it (a 7-qt crockpot works well).
Whisk the tomato paste into the beef broth and pour over the roast. Add the thyme. The meat does not need to be submerged.
Cover and cook on LOW for 8-10 hours or HIGH for 5-6 hours, until the roast pulls apart easily with a fork.
Remove the beef. Mix the cornstarch and cold water into a slurry, stir into the crockpot until thickened, discard the thyme stems, and return the beef. Set on high 5-10 minutes.
Pull the roast into tender pieces and serve with the potatoes and vegetables, spooning the gravy over everything.