In a large shallow bowl, spread the cornstarch then dredge beef in it. Place the steak in a large re-sealable plastic bag. Combine onion, garlic, brown sugar, soy sauce, water, carrots and ginger root in a bowl then pour mixture into the bag with the steak. Seal and refrigerate overnight. When ready, allow to thaw before emptying the bag into a slow cooker. Cover and cook for 4 to 5 hours on low.