Season chicken in a little salt and pepper.
Toss kway teow/horfun in fish sauce.
Heat pan on med high, then add oil.
Stir fry kway teow/horfun quickly with sugar. If kway teow is hard, add a splash of water to soften. Set aside.
In wok, bring chicken stock to boil.
Add garlic, galangal, lime leaves, chilli padi, and lemongrass. Allow to boil for 5 minutes.
Lower heat to med, add chicken, and mushrooms.
When chicken almost cooked through, season broth with fish sauce, lime juice, and sugar.
Add prawns and cook for a minute.
Fish out lime leaves, lemongrass, and galangal.
Add chye sim to broth.
Add egg while stirring constantly.
While stirring, drizzle in corn starch slurry until sauce thickens into a thick gravy.
Remove from heat and serve over prepared kway teow/horfun.
Garnish with chopped coriander, and serve immediately.