01 - Combine sliced beef with 1 tablespoon soy sauce, cornstarch, and black pepper in a bowl. Mix thoroughly and let rest for 10 minutes.
02 - In a small bowl, whisk eggs with milk or water and a pinch of salt until smooth.
03 - Heat 1 tablespoon vegetable oil in a large wok or skillet over medium-high heat. Stir-fry the beef until just browned, about 2 minutes. Remove from the pan and set aside.
04 - Add the remaining 1 tablespoon oil to the pan. Sauté 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 vegetables aside and pour in the beaten eggs. Gently scramble until just set.
06 - Add the chilled cooked rice to the pan. Stir thoroughly, breaking apart any clumps and mixing with vegetables and eggs.
07 - Return the beef to the pan. Add soy sauce, oyster sauce, sesame oil, and sugar if using. Stir-fry everything for 2 to 3 minutes until well combined and thoroughly heated.
08 - Remove from heat and stir in sliced green onions. Serve immediately while hot.