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