Slice the potatoes in to 1/2 length size. Clean the small onions and cut it into lengthy pieces. Fry the grated coconut, coriander powder and chilli powder without using oil. Saute it well till the mixture gets coffee brown colour. Finely grind the mixture without using water and set aside.
Put potatoes and small onions in a vessel and allow it to get half boiled. Then add the finely ground coconut mixture and keep it in medium flame for 6-7 minutes and set aside. Add tamarind pulp and salt and keep stirring.
Take a pan and season with mustard seeds, red and green chilies and curry leaves. Pour it in on top of the prepared Theeyal.