Trim fat from meat. Cut meat into 1-inch pieces.
Place meat in a 3- ½- to 4- ½-quart slow cooker.
Add squash, onions, and garlic. Stir in beef broth, tomato sauce, Worcestershire sauce, dry mustard, pepper, and allspice.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours. If using low-heat setting, turn to high-heat setting.
In a small bowl, combine cold water and cornstarch.
Stir cornstarch mixture and green beans into mixture in slow cooker.
Cover and cook about 15 minutes more or until thickened.