01 - Combine thinly sliced beef with 1 tablespoon soy sauce, cornstarch, and black pepper in a bowl. Mix well and set aside to marinate for 10 minutes.
02 - In a small bowl, beat the eggs with milk or water and add a pinch of salt.
03 - Heat 1 tablespoon vegetable oil in a large wok or skillet over medium-high heat. Add the marinated beef and stir-fry until just browned, about 2 minutes. Remove from the pan and set aside.
04 - Add the remaining 1 tablespoon of oil to the pan. Sauté finely chopped onion and minced garlic for 1 minute until fragrant, then add frozen mixed vegetables and stir-fry for 2 to 3 minutes until heated through.
05 - Push the vegetables to one side of the pan. Pour in the beaten eggs and scramble gently until just set.
06 - Add the cooked rice to the pan and stir all ingredients together, breaking up any clumps of rice.
07 - Return the cooked beef to the pan. Pour in soy sauce, oyster sauce, sesame oil, and sugar if using. Stir-fry everything for 2 to 3 minutes until well coated and heated through.
08 - Remove the pan from heat. Stir in sliced green onions and serve immediately while hot.