Begin by boiling a large pot of salted water.

Once boiling, add the egg noodles and cook according to the package instructions until al dente, about 5-7 minutes.

While the noodles are cooking, in a bowl, combine the sliced beef with soy sauce, oyster sauce, and cornstarch. Let the beef marinate.

In a large skillet or wok, heat the vegetable oil over medium-high heat.

Once hot, add the marinated beef in a single layer and cook for about 3-4 minutes without stirring.

Once browned, stir the beef for another minute until cooked through.

Add the sliced onion, minced garlic, broccoli, and bell peppers to the skillet and stir-fry for about 3-4 minutes until tender but still crisp.

Drain the cooked noodles and add them to the skillet with the beef and vegetables. Toss everything together to combine.

Season the dish with salt and pepper to taste, give it a final toss, and let it cook for an additional minute.

Remove from heat and serve hot.