1. Add hot water to the tamarind and extract thick tamarind juice.
2. wash the yam in water with which the rice was cleansed (this prevents the yam irritation in the throat)
3. Boil the yam in the pressure cooker upto 5 whistles, peel the skin and mash coarsely,
4. Heat oil in a pan (gingelly oil preferred), season with fenugreek seeds first and when it browns, add mustard seeds and allow to splutter.
5. Add red chillies, a little water and then the tamarind juice.
6. Add salt to taste and then the mashed yam and the slit green chillies.
7. Allow them to boil well and when thick remove from fire.

Note: Variation 1: You may add curry leaves when seasoning the fenugreek and mustard seeds.
If you want to avoid tamarind, follow the same method and instead of tamarind add lemon juice after removing from fire.