Pat the chuck roast completely dry and season all over with the salt and pepper.
Heat the oil in a large skillet over medium-high heat until shimmering. Sear the roast 3–4 minutes per side until deeply browned. Transfer to the slow cooker.
Pour 1 cup of the beef broth into the hot skillet and scrape up all browned bits from the bottom. Pour over the roast.
Sprinkle the onion soup mix evenly over the roast. Add the remaining broth, Worcestershire sauce, garlic, thyme, and bay leaves.
Cover and cook on LOW for 8–9 hours, or HIGH for 5 hours, until the beef is fork-tender.
Add the potatoes, carrots, and pearl onions during the last 2 hours of cooking on LOW.
Transfer the roast and vegetables to a platter and tent with foil. Discard the bay leaves.
Strain the cooking liquid into a saucepan and skim off the fat. Bring to a simmer.
Whisk the cornstarch and cold water together, then stir into the simmering liquid. Cook 2–3 minutes until thickened. Season to taste.
Slice or shred the beef, arrange with the vegetables, and spoon the gravy over the top.