Pat beef dry with paper towels. In a small bowl, mix salt, pepper, thyme, and rosemary. Rub all over the roast. Heat oil in a large skillet over medium-high heat. Sear the beef on all sides until deeply browned, about 3-4 minutes per side. Transfer to slow cooker.
Reduce skillet heat to medium. Add onion wedges and cook for 2 minutes until slightly softened. Add garlic and tomato paste, stir constantly for 30 seconds. Pour in a splash of beef broth to deglaze, scraping up browned bits. Transfer onion mixture to slow cooker.
Place carrot chunks and potatoes around and on top of the beef in the slow cooker. Pour in the remaining beef broth and Worcestershire sauce. The liquid should come about 1/3 up the sides of the roast, not covering it.
Cover and cook on LOW for 8-10 hours or on HIGH for 5-6 hours, until beef is fork-tender and reaches an internal temperature of at least 145°F (63°C). Do not uncover during cooking—keep the lid sealed.
When roast is done, carefully transfer beef (keep whole) and vegetables to a platter, tent with foil. Skim excess fat from juices in slow cooker, then whisk in the beurre manié (flour-butter paste) bit by bit. Turn heat to HIGH, cover and cook 15-20 minutes until thickened, stirring once. Season gravy with salt and pepper.
Slice the roast against the grain into 1/2-inch thick slices. Arrange on a serving platter with vegetables, then ladle gravy over the top. Serve immediately.