Slice the flank steak against the grain into ¼-inch-thick pieces, transfer to a bowl, and add marinade ingredients. Allow to marinate at room temperature for 15-30 minutes.

In a small mixing bowl, combine soy sauce, oyster sauce, dark soy sauce, Shaoxing wine, and sugar. Whisk until sugar dissolves and set aside.

Heat a large pan or wok over medium-high heat, add vegetable oil, and cook marinated beef in a single layer for 4-5 minutes until browned and cooked through.

In the same pan, add more oil if necessary, then toss in sliced onions and cremini mushrooms. Stir-fry for about 2 minutes until softened and onions are translucent.

Pour the prepared sauce into the pan with the sautéed vegetables and simmer for 1-2 minutes until it thickens slightly.

Return the cooked beef to the pan with chopped green onions, toss together for about 30 seconds, and then plate the stir-fry.