Pat the beef chuck roast dry with paper towels. Season all sides with kosher salt and black pepper.
Heat olive oil in a large skillet over medium-high heat. Sear the 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 skillet and sauté for 2-3 minutes until fragrant. Transfer onions and garlic to the crockpot, spreading them around the roast.
Pour the beef broth and Worcestershire sauce into the crockpot. Sprinkle dried thyme, dried rosemary, onion powder, and garlic powder evenly over the roast and broth.
Cover and cook on low for 8 hours, or until the beef is tender and easily shredded with a fork.
Remove the roast from the crockpot and place on a serving platter. Cover loosely with foil to keep warm.
Pour the cooking liquid into a saucepan through a fine mesh strainer to remove solids. Bring the strained liquid to a simmer over medium heat.
In a small bowl, whisk together cornstarch and cold water until smooth. Slowly whisk the cornstarch mixture into the simmering liquid. Cook, stirring constantly, until the au jus thickens slightly, about 2-3 minutes.
Serve the beef roast sliced or shredded with the warm au jus poured over or on the side for dipping.