Place the beef, onion, mushrooms, garlic, thyme and rosemary in the crock of a 4 to 5 quart slow cooker. Stir in 2 cups (500 mL) broth and the tomatoes. Set the heat to Low and cook for 6 to 7 1/2 hours (or up to 10 hours).
Turn the heat to High. Whisk the remaining broth with the cornstarch. Stir into the slow cooker until well combined; add the vegetables. Cover and cook for an additional 20 to 30 minutes or until heated through.
Meanwhile, prepare the noodles according to package directions. Spoon the hot stew over the noodles to serve.