In 4- to 5-quart slow cooker, mix steak and 1 tablespoon of the cornstarch until evenly coated. Gently stir in soy sauce, sherry, brown sugar, ginger and garlic. Top with water chestnuts.
Cover; cook on Low heat setting about 6 hours.
About 30 minutes before serving, in small bowl, blend 1/4 cup water and remaining 1 tablespoon cornstarch until smooth. Stir into beef mixture. Stir in thawed vegetables.
Increase heat setting to High; cover and cook 25 to 30 minutes longer or until vegetables are crisp-tender. Meanwhile, cook rice in 3 cups water as directed on package. Serve beef mixture over rice.