Mix the sauce: In a small bowl combine 1 tbsp light soy sauce, 1/2 tbsp dark soy sauce, 1/2 tbsp oyster sauce, 1/2 tsp sugar, salt and black pepper to taste, and 1/2 cup water. Stir until sugar is fully dissolved. Set aside next to the stove.
Whisk the eggs: Crack 3 eggs into a bowl and whisk until fully combined with no streaks remaining. Set aside next to the stove ready to pour in when needed.
Cook the onion and beef: Heat 1 tbsp cooking oil in a wide pan over medium heat. Add 1/4 sliced onion and sauté until softened and fragrant. Add 10 sliced beef rolls and spread evenly across the pan. Pour the sauce over the beef and onion and stir gently to coat. Place the lid on and simmer over medium heat until the beef is cooked through and the sauce has reduced to a glossy consistency.
Add the egg and set: Reduce heat to low. Pour the whisked egg evenly over the beef and onion. Place the lid on immediately and cook on low heat until the egg is about 80-90% set with just a slight wobble remaining in the centre. Take off the heat immediately. Residual heat will finish it perfectly.
Garnish and serve: Spoon steamed white rice into a bowl. Scoop the beef and egg mixture over the rice and spoon plenty of the glossy sauce over the top. Scatter white sesame seeds and finely chopped spring onion over everything. Serve immediately.