Peel the potatoes and cook in boiling salted water until just tender, then drain and cool. Cut into quarters lengthways and slice very thinly. Peel and finely chop the onion.
Heat the oil in a frying pan and add the onion. Fry gently until the onion has softened, then add the potato and fry a little more quickly until both are just beginning to brown. Draw the pan off the heat. Crack th