Start by slicing the sirloin steak into ½ inch thick strips. In a medium bowl, combine the steak strips with light soy sauce, dark soy sauce, rice wine vinegar, sugar, black pepper, and sesame oil. Mix well and let it rest for 10 minutes.
While the beef marinates, prepare the sauce. In a small bowl, mix together oyster sauce, rice wine vinegar, light soy sauce, dark soy sauce, brown sugar, cornstarch, and sesame oil until fully combined.
Heat a wok until very hot and add a drizzle of vegetable oil. Once the oil is slightly smoky, brown the beef in batches. Remove the beef and set it aside.
In the same wok, add the broccoli florets along with 2 tablespoons of water. Cover and allow the broccoli to steam cook for 5 minutes. Move the broccoli to the side, add a couple of teaspoons of sesame oil, and fry the minced ginger and garlic until fragrant. Toss them with the broccoli.
Add the cooked beef back into the wok and pour in the sauce. Gently toss to coat the broccoli and beef evenly. Add the roasted cashews.
Serve the beef stir fry with a bowl of delicious rice, garnished with green onion and toasted sesame seeds.