In a mixing bowl, combine the oyster sauce, low-sodium soy sauce, dark soy sauce, granulated sugar, black pepper, and Sriracha. Stir until the sugar dissolves and the mixture is well blended.

In a bowl, mix the thinly sliced beef with baking soda, garlic powder, and 1½ tablespoons of the prepared sauce. Let it marinate for 15-20 minutes.

Heat a wok or large skillet over medium-high heat and add a tablespoon of cooking oil. Once the oil shimmers, add the marinated beef and sear for 1-2 minutes until browned.

In the same skillet, drizzle in another tablespoon of oil and add sliced onion and Chinese broccoli. Stir-fry for about 2-3 minutes until the onion becomes translucent.

Add the pre-cooked flat rice noodles to the skillet with the sautéed vegetables, along with half of the prepared sauce. Toss gently for about 1-2 minutes.

Stir in the bean sprouts and chopped green onion, pouring in the remaining sauce. Mix everything thoroughly and cook for another 1-2 minutes.

Return the seared beef to the skillet, mixing everything together over high heat for a brief moment, let it sit for an additional minute before serving.