Pat the beef chuck roast dry with paper towels. Season all sides evenly with salt and black pepper.
Heat the 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 slow cooker.
Add the sliced onion and minced garlic to the slow cooker, spreading them around and on top of the beef.
In a medium bowl, whisk together the beef broth, soy sauce, Worcestershire sauce, dried thyme, dried rosemary, and bay leaf. Pour the mixture over the beef in the slow cooker.
Cover and cook on low for 8 hours, or until the beef is tender and easily shredded with a fork.
Remove the beef from the slow cooker and place it on a serving platter. Cover loosely with foil to keep warm.
Strain the cooking liquid through a fine mesh sieve into a saucepan, discarding solids and the bay leaf.
In a small bowl, mix the cornstarch and cold water until smooth. Whisk the cornstarch slurry into the saucepan with the strained cooking liquid.
Bring the liquid to a simmer over medium heat, stirring frequently until it thickens slightly, about 3-5 minutes. Remove from heat.
Slice or shred the beef as desired and serve with the au jus drizzled over the top or on the side for dipping.