1. Partially freeze beef; slice it thinly into bite-sized strips. Combine with soy sauce and marinate at room temperature for 15 minutes.
2. Heat wok or large nonstick skillet. Pour in chicken stock and stir-fry ginger root for 30 seconds. Add beef and stir-fry for 3 minutes until browned.
3. Whisk cornstarch and bouillon granules into 1/4 cup water. Add to wok. Cook and stir until thickened.
4. Stir in spinach and water chestnuts. Cook, covered, for 2 minutes.