Heat the oil in a deep frypan or wok over medium heat. Add the onion and cook, stirring, for 2-3 minutes until softened but not coloured. Add garlic and chilli and cook for a further minute, then add curry paste and cook, stirring, for 1-2 minutes until fragrant. Add potatoes, coconut milk and 400ml water. Bring to the boil, then reduce heat to low and simmer for 10 minutes until potatoes are almost tender.