1. Combine beef, oyster sauce, soy sauce, salt, and ground black pepper. Mix and marinate for 30 minutes.
2. Heat a wok and pour-in cooking oil.
3. Sauté garlic, ginger, and chili.
4. Stir-fry spinach then place in a serving plate and set aside.
5. Using the same wok with remaining oil, stir-fry the marinated beef for 5 to 6 minutes or until the color turns brown.
6. Arrange the beef on top of the spinach. Serve hot.