slice potatoes lengthwise into quarters. then cut them crosswise into 1-inch pieces.
make tadka: heat oil in large wok over high heat. when oil begins to smoke, add cumin seeds, covering pan with lid. after seeds stop sputtering, add chile. when chiles are well toasted, add turmeric and briefly stir. add potatoes and salt, toss well, cover, and leave on medium-high heat until potatoes are slightly toasted, about 4 minutes.
uncover and add coriander seeds and toss well again. continue to heat uncovered, tossing occasionally, until potatoes are crusty and well browned, 6-8 minutes. serve warm.