To avoid itchiness carefully slice off the outer (thick, rugged & muddy) skin of the yam before washing it. Cut it into cubes and then wash the cubes and keep them aside.
In a large wok or kadhai heat the oil and add the mustard. When the mustard stops spluttering add the curry leaves and the crushed garlic and saute for a few seconds. Next add the onions and fry on a medium low heat till they turn light golden in colour.
Add the grated coconut and continue to fry till you get a dryish mixture and a nice aroma from the roasted coconut.
Add the vegetable masala powder and fry for a few minutes taking care to see that it doesnt get burnt.
Add the tamarind juice and salt to taste and the yam pieces. Pour enough water to cover the cubes - not too much.
Cover the pan and let the yam cook on a medium heat till the pieces turn tender. Stir in between to avoid scorching. Sprinkle more water if required and adjust the spice powder, salt & tamarind if required. When done, remove from heat
Serve hot with rice or chapathis