Peel and chop the yam into small cubes. Then cook it in turmeric powder and water in a pan for few minutes or until the yam pieces get half cooked. Drain the water and keep aside.
Now heat oil in a pan and splutter the urad dal and mustard seeds, then add the curry leaves and cooked yam pieces to it and saute for few minutes. Add the coriander and chilly powder and some salt. Toss until the yam pieces get well coated with the spice powder.
Add some water and simmer until the water dries up.