Place the dried rice noodles in a large heatproof bowl and cover with boiling water. Let soak for 8-10 minutes (or according to package directions) until pliable but still firm (al dente). Drain and rinse with cold water to stop cooking. Set aside.
In a medium bowl, toss the sliced beef with 1 tablespoon of soy sauce and 1 tablespoon of cornstarch until well coated.
In a small jar or bowl, whisk together the beef broth, oyster sauce, brown sugar, sesame oil, remaining 3 tablespoons of soy sauce, and remaining 1 tablespoon of cornstarch. Set aside.
Heat 1 tablespoon of vegetable oil in a wok or large skillet over high heat. Add the beef in a single layer and sear for 1-2 minutes per side until browned. Remove beef from the pan and set aside.
Add the remaining 1 tablespoon of oil to the pan. Add the onion wedges and green beans. Stir-fry for 3-4 minutes until the beans are crisp-tender.
Stir in the minced ginger and garlic, cooking for 30 seconds until fragrant.
Add the cooked noodles and the prepared sauce mixture to the pan. Toss continuously for 1-2 minutes until the sauce thickens and coats the noodles.
Return the cooked beef to the pan and toss to combine.
Remove from heat and stir in the roasted cashews just before serving.