Heat the oil in a large skillet over medium-high heat and add the onion
. Cook for 3 minutes and add the mustard seeds
, turmeric, cardamom
seeds, curry leaves
, cumin seeds
, and salt. Stir and cook for another minute. Add the potatoes, sour cream, tomatoes
, and water, and stir well. Turn the heat to low and cook until the potatoes are tender, about 15 to 18 minutes. Serve the potato salad
with rice or rotis.