This is a filling one-pot soup for dinner. Use time while the noodles hydrate to prep all the fresh ingredients.
Per serving:
In small bowl, whisk soy sauce with sesame oil, vinegar, sugar, garlic and chilli flakes (if using).
Drain noodles, then divide among 4 large soup bowls. Divide cabbage, then pile over top. Set aside.
Heat oil in large wok or frying pan over medium-high heat. When hot, add chicken, onion and mushrooms. Stir-fry until chicken is cooked through, about 2 minutes. Pour in soy sauce mixture, then add bok choy. Stir-fry until bok choy wilts, about 1 minute. Divide mixture among soup bowls.
Return frying pan to cooktop and add stock and water. Bring to a boil, stirring often, then pour into soup bowls.