Marinate the Steak:
In a mixing bowl, combine soy sauce, oyster sauce, rice vinegar, sesame oil, garlic, and ginger.
Add the thinly sliced steak to the marinade and toss to coat. Let it marinate for at least 15 minutes, or up to 2 hours for more flavor.
Prepare the Noodles:
Cook the chow mein noodles according to package instructions. Drain and set aside.
Cook the Vegetables:
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the broccoli florets, sliced onion, and red bell pepper. Stir-fry for 3-4 minutes until the vegetables are crisp-tender. Remove from the skillet and set aside.
Cook the Steak:
In the same skillet, heat the remaining tablespoon of vegetable oil over high heat. Add the marinated steak in a single layer and cook for 2-3 minutes on each side until the steak is browned and cooked to your desired doneness. Remove from the skillet and set aside.
Make the Sauce:
In a small bowl, whisk together soy sauce, oyster sauce, hoisin sauce, cornstarch, beef broth, and sesame oil until smooth.
Combine Everything:
Return the vegetables and steak to the skillet. Add the cooked noodles and pour the sauce over everything.
Toss to coat all the ingredients evenly with the sauce. Cook for another 2-3 minutes until everything is heated through and the sauce has thickened.
Serve:
Garnish with sliced green onions and serve immediately.