Pat the roast dry and season on all sides with the salt and pepper. Heat 1 tablespoon of the oil in a large skillet or Dutch oven over medium-high heat. Place the roast in the pan and let sear without disturbing until it forms a nice crust on the bottom, about 4 minutes. Turn and cook on the second side until brown, 3 to 4 minutes more. Transfer to a 6-quart or larger slow cooker.

Add the remaining 1/2 tablespoon oil to the skillet. Add the onion and cook until it's beginning to color, 2 to 3 minutes. Stir in the garlic and let cook 30 seconds, until fragrant.

Slowly pour in the broth, using a wooden spoon to scrape up any browned bits stuck to the pan. Add the rosemary sprigs, tomato sauce, and Worcestershire. Stir to combine , partially cover the pot, and bring to a rapid simmer. Let simmer 30 seconds, then remove from the heat.

Arrange the potatoes and carrots on top of the beef in the slow cooker. Pour the pan mixture over the top, then sprinkle the chopped rosemary all over.

Cover the slow cooker and cook the roast on LOW until the roast is fall-apart tender and the vegetables are tender, about 8 hours. With a large spoon, carefully transfer the roast and vegetables to a serving platter (at this point, the beef will be falling apart). Discard any large pieces of fat from the beef.

To make a gravy, strain the cooking liquid from the slow cooker through a sieve and into a saucepan (if you like, you can pass it through a fat separator also). In a small bowl, briskly whisk together the slurry of 2 tablespoons cornstarch and 2 tablespoons water until smoothly combined. Add to the pot with the cooking liquid and whisk to smoothly combine. Bring to a simmer over medium-high heat. Continue cooking, whisking constantly, until the gravy is slightly thickened, about 1 minute.

Spoon the gravy over the beef and vegetables. Sprinkle with parsley. Serve hot, ensuring each serving gets a little bit of everything.