This recipe is from Charmain Solomon's "The complete Asian cookbook", which is utterly excellent and highly recommended.
I've found it works equally well if you peel and dice the potatoes before cooking - they also cook quicker! It's a good idea to cook the potatoes a bit in advance and let them cool as they're then less likely to crumble when you add them to the onions and spices.
2 medium onions
1 tbsp ghee or oil
1/4 tsp black mustard seeds
1/2 tsp ground turmeric
1/2 tsp chilli powder
1 tsp salt
Boil the potatoes, peel and dice (about 1 cm cubes). Chop onions finely. Heat ghee or oil in a saucepan and fry the mustard seeds until they pop. Add onions and continue frying on low heat until they are soft and golden brown. Add turmeric and chilli powder and stir, then add potatoes, sprinkle salt over and toss gently to mix well together.