Cut chicken into small strips and mix with fish sauce in a bowl. Cover and set to one side.
Trim and dice all the veg. Heat 3 tablespoons oil in a large heavy pot and cook all veg except the broccoli and mushrooms till is starts to soften. Add stock and coconut milk and lemon grass. Cover, bring to the boil and reduce heat. Simmer for 15 minutes.
Meanwhile heat 2 tablespoons oil in a frying pan, add sugar and let caramelise. Add mushrooms and fry whilst stirring till they release liquid.
Add curry paste, shrimp paste, garlic, ginger, broccoli and the mushrooms to the veg and bring to the boil. Season with all purpose seasoning and add more curry paste if desired.
Add chicken and cook over look heat for 10 minutes.