Pressure cook the potatoes. Slice the onion and ginger to thin longitudinal strips. Slit the chillies
Extract milk from the coconut. To extract the first milk add ½ cup of warm water and grind the coconut. Extract the milk using a strainer. Keep aside.
Next add 1 more cup of warm water to the squeezed coconut and strain the milk. This will be the second extract. Keep aside.
In a sauce pan add the sliced onions, ginger and green chillies. Pour two cups of water on the squeezed coconut and strain the third coconut milk directly into the sauce pan. Add salt as needed and cook for about 5 minutes.
Once the onion gets cooked, mash and add the cooked potatoes. As the mixture starts boiling add the second extracted coconut milk and cook for a few more minutes. You can add water here if you feel the ishtu is getting thick.
Simmer and add the first extracted coconut milk. Adjust salt and switch off just before it starts boiling.
Add the curry leaves and coconut oil. Serve with idli, dosa, idiappam, aapam, flatbreads or even plain rice.