Pat the roast dry and season with salt and pepper. Brown it in olive oil in a large skillet for about 4 minutes per side, then put it in a 6-quart slow cooker.
Cook the onion in the skillet for 5 minutes. Add the garlic and tomato paste and cook for 1 minute.
Pour in the broth, balsamic, honey and Worcestershire. Scrape up the browned bits and bring to a simmer, then pour everything around the roast and add the rosemary.
Cover and cook on low for 8 hours, until the beef yields easily to a fork. Transfer the roast to a board and discard the rosemary stems.
Skim the liquid. Stir together cornstarch and water, add it to the cooker and cook on high for 10 minutes. Pull the beef into large pieces and coat it with the balsamic gravy.