In a mixing bowl, combine the cubed chicken with 1 tsp white pepper, 1 tbsp soy sauce, 1 tbsp cornstarch, and a pinch of salt; toss to coat and marinate for at least 30 minutes.
Cook the rice noodles according to package instructions, drain, rinse with cold water if directed, and set aside.
Heat the cast iron pan over medium-high heat and add 2 tbsp oil.
Add the minced garlic and sauté about 1 minute until fragrant.
Add the marinated chicken and cook 3–4 minutes, stirring occasionally, until mostly cooked through.
Add the broccoli florets and sliced onion, and cook for about 2 minutes until vegetables are crisp-tender.
Reduce heat slightly and add the cooked noodles; pour in 1 tbsp soy sauce, 1 tbsp Worcestershire sauce, 1 tbsp rice vinegar, 1/2 tsp white pepper, and salt to taste.
Toss everything together until the noodles and chicken are evenly coated and heated through, about 1–2 minutes.
Remove from heat, sprinkle with chopped green onions and sesame seeds, and serve warm.