In a medium bowl, combine the thinly sliced beef with the 2 tbsp light soy sauce, Shaoxing wine, 1 tsp cornstarch, and black pepper. Mix well until the beef is evenly coated. Let it marinate for at least 10-15 minutes while you prepare the other ingredients.

In a small bowl or measuring cup, whisk together all the sauce ingredients (light soy sauce, dark soy sauce, oyster sauce, sesame oil, sugar, water, and 1 tsp cornstarch) until the sugar and cornstarch are fully dissolved. Set aside. Thinly slice the yellow onions and separate the white and green parts of the scallions.

Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat. Add the sliced yellow onions. Cook, stirring occasionally, for 6-8 minutes until they are soft, fragrant, and starting to caramelize to a light golden color. Transfer the onions to a plate and set aside.

Increase the heat to high and add the remaining 1 tablespoon of oil to the now-empty skillet. Add the marinated beef in a single layer, working in batches if necessary to avoid overcrowding. Let it sear undisturbed for 1 minute, then stir-fry for another 1-2 minutes until the beef is just cooked through and browned. Remove the beef to the plate with the onions.

Reduce the heat to medium. Add the white parts of the scallions to the skillet and stir-fry for 30 seconds until fragrant. Give the prepared sauce a quick stir (the cornstarch may have settled) and pour it into the skillet. It will bubble and thicken immediately. Return the cooked beef, caramelized onions, and any accumulated juices to the skillet. Add the green parts of the scallions. Toss everything together for 1-2 minutes until the sauce coats every piece and the scallions are just wilted.

Remove the skillet from the heat. Spoon the hot Bœuf Sauté aux Oignons over steamed rice in individual black bowls. Garnish with a sprinkle of toasted sesame seeds if desired. Serve immediately with chopsticks.