Pat the beef chuck roast dry, and season with salt and pepper. In a large saute pan, heat the olive oil, and brown the beef on all sides (about 3 minutes per side).
While the beef is browning, layer the onions, carrots, garlic, rosemary and mushrooms in the bowl of a slow cooker. Once the beef is browned on all sides, place the beef on top of the vegetables in the slow cooker.
Deglaze the saute pan with the beef broth, making sure to scrape up any dark spots from the pan. Pour the broth in the slow cooker.
Pour the diced tomatoes over the beef and tuck in a dried bay leaf.
Cover and cook on low for 7 hours.
Remove the beef from the slow cooker using kitchen tongs, and set on a carving board. Shred or slice, and serve with a starchy side dish. Or, you can remove the sprig of rosemary and the bay leaf and blend the vegetables into a thick sauce that can be used as a gravy.