Peel and cut the yam into half centimeter sized cubes. Keep them in salted water for about half an hour. Drain well and dry on an absorbent cloth. Heat sufficient oil in a kadai and deep-fry the yam cubes till golden and crisp.
Drain on absorbent paper and set aside. Heat one tablespoon of oil in a non stick pan and sauté red chillies, mustard seeds, fenugreek seeds, asafoetida and turmeric powder till fragrant, taking care that the spices do not burn. Grind to a fine paste with the tamarind and a little water.
Mix the ground masala with salt and fried yam. Add three-fourth cup water so that the yam pieces soak properly in the masala paste. Let it stand for two to three hours before serving it.