Pat the beef chuck roast dry with paper towels. Season all sides with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat. Sear the beef roast on all sides until browned, about 3-4 minutes per side. Transfer the roast to the crockpot.
Add the sliced onion and minced garlic to the crockpot, spreading them around and on top of the roast.
Pour the beef broth and Worcestershire sauce over the roast. Sprinkle the dried thyme, dried rosemary, and add the bay leaves.
Cover the crockpot and cook on low for 8 hours, or until the beef is tender and easily shredded with a fork.
Once cooked, remove the roast and keep warm. Discard the bay leaves.
Pour the cooking liquid into a saucepan and bring to a simmer over medium heat.
In a small bowl, mix the cornstarch and cold water to create a slurry. Slowly whisk the slurry into the simmering cooking liquid.
Continue to cook and stir until the au jus thickens slightly, about 2-3 minutes.
Slice the beef roast against the grain and serve with the au jus sauce drizzled over the top.