Season beef all over with 3 ½ tsp. salt and a few grinds of pepper. In a large, heavy skillet over medium-high heat, heat oil. Cook beef, undisturbed, until bottom is browned, about 3 minutes. Turn and continue to cook, undisturbed, until other side is browned, about 3 minutes more. Transfer to a plate.
In a slow cooker, whisk broth, tomato paste, Worcestershire, and cornstarch. Add potatoes, carrots, onion, garlic, and rosemary; season with remaining 1 tsp. salt and a few grinds of pepper.
Nestle beef into vegetables. Cover and cook until beef is fork-tender, about 5 hours on High or 8 hours on Low. Transfer roast to a platter and thinly slice against the grain. Transfer vegetables to a platter. Arrange beef on top. Skim fat from extra juices in slow cooker and drizzle over. Serve warm.
