Heat the oil in a wok over a medium flame. When hot, add the mustard seeds. When the seeds begin to pop, add the Cayenne pepper (or chili powder), turmeric and salt. Stir around for 1 minute then add the potatoes and stir fry for 5 minutes at which time they should be brown/yellow and coated in seeds.
Reduce the heat to low, cover the wok and allow to cook for a further 5 minutes.