Firstly, boil the potatoes, cool them, peel and mash with your hands roughly. Meanwhile heat 1+1/2 cups of water in a separate pan.
Next, add oil to a Kadhai, heat it at medium flame. Add mustard seeds, once it sizzles add cumin. Saute them for just few seconds.
Next, add curry leaves, minced ginger and chilies. Saute for a while, add asafoetida.
After that add turmeric powder, red chilli powder and salt. Stir continuously and add hot water to it.
Now add mashed potatoes and sugar, let it come to a rolling boil. Now simmer the curry and cook it for 2-3 minutes more.
Lastly, add lemon juice and mix with light hands. The gravy would start to thicken after a while.
Switch off the flame. Taste the curry and adjust the salt and sugar accordingly. The curry should be spicy, tangy and sweet.
Garnish with coriander leaves and serve hot.