Heat the oil in a wok over a moderate heat. Add the onion and stir-fry for 4–5 minutes, until they are slightly golden. Remove using a slotted spoon and set aside.
Pour the coconut milk and stock into the wok and increase the heat to bring the liquid quickly to the boil. Add the curry paste, sugar and fish sauce and mix until combined.
Add the potatoes, carrots and fried onion, lower the heat and simmer for 10 minutes. Add the cauliflower and peas and cook for a further 10 minutes, by which time all the vegetables should be tender.
Tip into a large serving bowl and top with a scattering of coriander.