Combine the tomatoes and their juices, the beef broth, ketchup, brown sugar, Worcestershire sauce, prepared mustard, garlic, soy sauce and red pepper flakes in a bowl. Place the roast in the slow cooker and top with the tomato mixture.
Add the onions, bell pepper and celery to your slow cooker, on top of the roast and tomato mixture.
Secure the lid and cook on low for five to six hours or until the meat becomes fork-tender. Editor's Tip: The internal temperature for slow-cooked chuck roast should read around 195°F on your instant-read meat thermometer. Texture-wise, the meat should be fork-tender.
Take the roast out of the slow cooker and set it on a plate. Strain the cooking juices into a bowl, and set the vegetables aside. Pour the juices into a small pot, skim off and discard any fat floating on the surface. Whisk the cornstarch and water in a small bowl, then mix it into the cooking juices. Bring to a boil over medium-high heat, occasionally stirring until the gravy has thickened, about one to two minutes. Serve the roast and vegetables with a generous splash of gravy.
