In a 2-cup (480 ml) liquid measuring cup, combine the water, cornstarch, soy sauce, garlic, brown sugar, and white pepper.
In a medium bowl, season the steak strips with salt. Pour half of the sauce over the meat and set the rest aside for later. Mix the sauce with the steak, cover with plastic wrap, and marinate in the refrigerator for 30 minutes to 1 hour.
In a wok or deep skillet over medium-high heat, heat 2 tablespoons of cooking oil, then add the par-cooked noodles. Cook for 1-2 minutes, allowing the noodles to crisp, then flip and cook for another 1-2 minutes. Transfer to a plate and set aside.
In the same wok, add 1 tablespoon of cooking oil and the marinated beef. Cook the beef for 3-4 minutes, then flip and cook for another 2 minutes, or until slightly crispy. Remove from the wok and set aside.
In the same wok, heat the remaining tablespoon of oil and add onion and broccoli. Season with salt, then stir and cook for 3-4 minutes, or until the onions are translucent.
Return the steak to the wok, followed by the crispy noodles and sesame oil. Mix well and pour in the reserved sauce. Toss gently and cook for another 2-3 minutes, or until everything is well-combined. Garnish with sliced scallions.