Combine the tomatoes (with their juices), bell peppers, onion, oregano, garlic, and ½ teaspoon each salt and black pepper in a 4- to 6-quart slow cooker. Season the beef with 1 teaspoon salt and ½ teaspoon black pepper; place on top of the tomato mixture. Cover and cook on low for 7 to 8 hours or on high for 4 to 5 hours.
Shred the meat into large chunks. Spoon off any excess fat from the liquid.
Twenty minutes before serving, cook the polenta according to the package directions. Serve the polenta topped with the beef stew and basil.
* Total Time: 4 hours, 20 minutes to 8 hours, 20 minutes