Pat the beef chuck roast dry with paper towels. Season both sides evenly with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat. Once hot, sear the roast for 4-5 minutes on each side until browned. Remove from skillet and set aside.
Place the sliced onion, minced garlic, carrots, and celery in the bottom of the slow cooker.
In a small bowl, whisk together beef broth, tomato paste, dried thyme, dried rosemary, Worcestershire sauce, and bay leaves.
Place the seared roast on top of the vegetables in the slow cooker. Pour the broth 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.
Carefully remove the roast and vegetables from the slow cooker and place on a serving platter. Cover with foil to keep warm.
Remove and discard the bay leaves from the cooking liquid. Pour the liquid into a saucepan over medium heat.
In a small bowl, mix the all-purpose flour with water until smooth to create a slurry.
Gradually whisk the slurry into the saucepan with the cooking liquid. Cook, stirring constantly, until the gravy thickens, about 3-5 minutes.
Slice the roast beef against the grain and serve with the cooked vegetables and gravy.