For filling, in a large skillet cook pork, ginger, and garlic for 3 to 5 minutes or until meat is no longer pink; drain off any fat. Add cabbage, water chestnuts, carrot, and onion. Cook and stir 2 minutes more. In a small bowl stir together soy sauce, cornstarch, sugar, and salt. Add to skillet. Cook and stir until thickened and bubbly. Remove from heat and cool slightly.