Ingredients:

Potato                           :  1 big
Onion                           : 1 big
Green chillies               : 2 slit lengthwise
Ginger                          : 1"pc
Coconut milk               : 1 cup thin milk
Thick coconut milk     :  1/2 cup
Salt to taste
Coconut oil                 : 1 tbsp
Curry leaves               : 1 sprig


Method:

Cook whole potato in a pressure cooker adding 1 cup of water. Cool it and remove the skin and slightly mash it and keep aside.
Slice the onion. Cut ginger into thin long pieces and slit the green chillies lengthwise.
How to make coconut milk:
If you are taking fresh coconut milk, then grate half of a big coconut and squeeze it for thick coconut milk(onnam pal). It will be very less. keep it separately.
Then grind the squeezed coconut in a mixer adding 1 cup of warm water and filter it, this is the thin coconut milk(randam paal).
Nowadays you can easily buy fresh cocnut milk or coconut milk powder from the market. If you are using coconut milk powder then take 1 cup of warm water and add 3 tables spoons of coconut milk powder( consider this as randam pal) Take 1/2 cup of warm milk and add 1 full tbsp of coconut milk powder and this will be thick(onnam pal)
Cook onion,ginger and green chillies in the thin coconut milk for about 5 mins. Then add salt and mashed potato to this. Boil for a minute and finally add the thick coconut milk. Before boiling remove from fire. Add 1tbsp of coconut oil and curry leaves. It goes well with idli, dosa, appam etc.