Heat the oil in a large skillet over medium-high heat and add the onion and garlic. 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.
*Can be found at specialty Asian and Indian markets.