Heat mustard seeds and cumin seeds in a hot dry skillet over high heat, swirling the pan, until the seeds start to pop.
Reduce the heat to medium; add the chilies, onions and a small amount of water; cook until the onion is tender (adding more water if necessary to prevent sticking).
Add the potatoes, ground cumin, turmeric, coriander, cayenne pepper and salt (to taste), stirring to mix well.
Add 2/3 cup water; cover, reduce the heat and cook for about 20-25 minutes, stirring occasionally (until the potatoes are tender and the spices are absorbed).
Add a small amount of water at a time if all liquid evaporates.
Fold in the chopped cilantro just before serving.