Prepare the Beef:

In a bowl, combine thinly sliced beef, cornstarch, salt, and pepper. Toss to coat evenly. Set aside to marinate for about 10 minutes.

Prepare the Sauce:

In a separate bowl, whisk together soy sauce, brown sugar, hoisin sauce, chili garlic sauce, ginger, garlic, and water or beef broth. Set aside.

Cook the Noodles:

Cook noodles according to package instructions until just al dente. Drain and set aside.

Cook the Beef:

Heat vegetable oil in a large skillet or wok over high heat. Add the marinated beef in a single layer, allowing it to sear and brown without stirring for about 1-2 minutes. Then stir-fry until nearly cooked through. Remove beef from the skillet and set aside.

Sauté Vegetables:

In the same skillet, add a little more oil if needed, then sauté onion and red bell pepper until they start to soften, about 3-4 minutes.

Combine Ingredients:

Return the beef to the skillet. Pour the prepared sauce over the beef and vegetables. Stir well to combine and cook for another 2-3 minutes until the sauce is bubbling and has thickened slightly.

Add Noodles:

Add the cooked noodles to the skillet. Toss everything together, ensuring the noodles are well coated with the sauce and ingredients are evenly distributed.

Garnish and Serve:

Remove from heat. Sprinkle with chopped green onions and sesame seeds before serving.