Season the beef chuck roast evenly with salt and black pepper on all sides.
Heat olive oil in a large skillet over medium-high heat. Sear the roast for 3-4 minutes on each side until browned. Remove from heat.
Place the sliced onion, minced garlic, carrots, and celery at the bottom of the slow cooker.
Put the seared beef roast on top of the vegetables in the slow cooker.
In a small bowl, whisk together beef broth, tomato paste, dried thyme, dried rosemary, and bay leaves. Pour this mixture over the roast and vegetables.
Cover and cook on low for 8 hours, or until the beef is tender and easily shredded with a fork.
Remove the roast and vegetables from the slow cooker and transfer to a serving platter. Cover with foil to keep warm.
Discard the bay leaves from the slow cooker. Pour the cooking liquid into a saucepan.
In a small bowl, mix cornstarch and cold water until smooth. Whisk the cornstarch mixture into the cooking liquid.
Heat the saucepan over medium heat, stirring constantly, until the gravy thickens, about 3-5 minutes.
Serve the beef roast sliced or shredded with the cooked vegetables and pour the gravy over the top.