Heat the oil in a large, deep pan over medium heat.

Add the garlic and ginger, and cook for about a minute.

Add the onions and cook for a further 4 minutes or until softened. Increase the heat to medium-high and add the chicken until lightly browned.

Add the fish sauce and cook for 30 seconds.

Add the stock and lemongrass. Bring to a boil, then reduce to medium heat and cook for 20 minutes.

Add the chillies and papaya or chayote or potato, whichever you are using, and cook until the papaya or chayote or potato are tender.

Stir in the coconut water and cook for a further 5 minutes.

Correct seasonings, if needed.

Discard the lemongrass.

Transfer to a serving dish and garnish with coriander.