Boil the potatoes, peel the skin and mash slightly it into small pieces and set aside
In a small bowl add the besan / gram flour, 1/4 cup water and mix it well and set it aside for further use
Heat pan with oil add mustard seeds, cumin / jeera once it crackles add curry leaves and saute it for a minute
Add ginger garlic paste and saute untill it turns aromatic
Add sliced onions and fry them till they turn translucent
Now add slit green chili and mix it
Add finely chopped tomatoes and cook it till it tuns mushy
Add turmeric powder and mix it
Add the besan mixture and mix it well
Add the smashed potatoes, salt and mix it well
Add more water and taste test for salt and cover and cook for 3-4 minutes on medium to high flame or until starts to boil Note : Potato sagu should be in medium consistency should not be more thick or thin Finally garnish it with coriander leaves and switch off the flame
Serve it hot with poori, chappati, idli, dosa or rava idli