01 - Combine beef slices with soy sauce, cornstarch, sesame oil, and black pepper in a bowl. Mix thoroughly and let marinate for at least 10 minutes at room temperature.
02 - Whisk together oyster sauce, light soy sauce, water, sugar, and cornstarch in a small bowl until completely smooth. Set aside for later use.
03 - Heat 1 tablespoon vegetable oil in a large wok or skillet over medium-high heat. Add marinated beef and stir-fry for 1-2 minutes until just browned on the outside. Remove beef from wok and set aside on a plate.
04 - Add remaining 1 tablespoon oil to the wok. Add garlic, ginger, onion, and bell pepper. Stir-fry for 2 minutes until fragrant and vegetables begin to soften.
05 - Add snow peas to the wok and continue stir-frying for 1 minute until bright green but still crisp.
06 - Return the beef to the wok. Pour in the prepared sauce mixture. Stir-fry everything together for 1-2 minutes, tossing continuously until sauce thickens and coats the beef and vegetables evenly.
07 - Remove from heat immediately and serve hot over steamed jasmine rice.