In a bowl, marinate the sliced beef with soy sauce, oyster sauce, and cornstarch for 10 minutes.
Cook the egg noodles according to package instructions, then drain and set aside.
Heat vegetable oil in a large skillet over medium-high heat.
Add the marinated beef to the skillet and stir-fry until browned (about 5 minutes).
Remove the beef from the skillet and set it aside.
Add sliced onions and minced garlic to the skillet, sautéing until fragrant.
Pour in the beef broth and bring to a simmer.
Add the cooked noodles and stir until well combined.
Return the beef to the skillet, mixing it with the noodles and sauce.
Finally, add bean sprouts, seasoning with salt and pepper, and cook for an additional 2-3 minutes.