Bring a saucepan of water to the boil and blanch the potatoes about 8 minutes or until they are tender but not mushy. Drain.
Heat the vegetable oil in a frypan. Fry the onion 3 minutes then add garlic, chilli, coriander and cumin. Fry one more minute then add the potatoes and salt to taste. Saute about 5 minutes. Place in a bowl and let cool.
When cool, add the avocado and prawns. Add the lime juice and mix gently. Chill and serve!