Whisk together oyster sauce, chili garlic sauce, salt, pepper, and water in a small bowl. This flavorful sauce will coat your noodles and vegetables perfectly. Set it aside while preparing the beef.
In a larger bowl, combine thinly sliced flank steak with garlic, cornstarch, egg yolk, salt, and pepper. Toss everything together until the beef is fully coated. This ensures a tender, slightly crispy texture when cooked.
Heat 1 tablespoon of peanut oil in a wok or large skillet. Stir-fry half of the beef for 3 minutes until browned. Cook in batches to prevent sticking, adding oil as needed, and set cooked beef aside.
Add the remaining oil to the skillet, then sauté onions and mushrooms for 5 minutes until softened and lightly browned. This builds the base flavor for the stir fry.
Add bell peppers, broccoli, and 1/3 cup of water to the skillet. Steam and cook for about 8 minutes until the vegetables are tender-crisp. Avoid overcooking to maintain texture and color.
Add the cooked beef back into the skillet with noodles and sauce. Toss well until everything is evenly coated and heated through. Serve immediately for the best flavor and texture.