Marinate the beef by combining sliced beef with soy sauce, cornstarch, and vegetable oil in a bowl. Let it sit for 15 to 30 minutes.

Prepare the sauce by whisking together the soy sauce, oyster sauce, hoisin sauce, sugar, rice vinegar, sesame oil, garlic, and ginger in a separate bowl.

Cook the noodles in a pot of boiling water according to package instructions, usually around 4-6 minutes. Drain and toss with oil to prevent sticking.

Stir-fry the marinated beef in a skillet over medium-high heat for 2-3 minutes until browned and cooked through. Remove and set aside.

In the same skillet, stir-fry the bell pepper and broccoli for 3-4 minutes until bright and tender-crisp.

Combine the beef, vegetables, and sauce in the skillet. Stir together over medium heat for 2 minutes, then add the cooked noodles and mix until well combined.

Serve immediately, garnished with green onions and sesame seeds.