Combine 2 tablespoons oyster sauce, soy sauce, sherry, garlic powder, salt, and ground black pepper in a large mixing bowl. Stir.
Add the beef slices. Continue to stir until the beef gets coated. Marinate the beef for 30 minutes.
Heat the cooking oil in a wok or frying pan.
Once the oil gets hot, put the onion and then cook for 30 seconds.
Add the marinated beef including the marinade. Stir-fry for 4 to 5 minutes.
Add the snap peas and remaining oyster sauce.
Combine cornstarch and water. Mix well. Pour the mixture in and then continue to cook while stirring for 3 to 4 minutes.
Transfer to a serving plate and serve with rice.
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