Pat the beef chuck roast dry with paper towels. Season all sides evenly with kosher salt and freshly ground black pepper.
Heat the olive oil in a large skillet over medium-high heat. When hot, add the beef roast and sear for 4-5 minutes on each side until browned. Remove the roast and set aside.
Place the sliced onion and minced garlic in the bottom of the crockpot. Lay the seared roast on top of the onions and garlic.
In a medium bowl, whisk together the beef broth, Worcestershire sauce, dried thyme, dried rosemary, and bay leaf. Pour this mixture over the roast in the crockpot.
Cover and cook on low for 8 hours, or until the beef is tender and easily shredded with a fork.
Once cooked, carefully remove the beef roast from the crockpot and transfer to a serving platter. Tent with foil to keep warm.
Remove the bay leaf from the crockpot liquid. Pour the cooking juices into a medium saucepan and bring to a simmer over medium heat.
In a small bowl, mix the cornstarch and cold water until smooth to create a slurry. Gradually whisk the slurry into the simmering cooking juices.
Continue to cook and whisk until the au jus thickens slightly, about 2-3 minutes. Remove from heat.
Slice the beef roast against the grain and serve with the warm au jus poured over or on the side for dipping.