Pressure cook potatoes with little turmeric for 4-5 whistles till they are cooked .Once they are cool ,deskin ,mash and keep aside . you need not mash to paste . a few chunks here and there is fine .
Heat oil in a kadai . Add mustard , urad dal and chana dal.
Once they splutter add curry leaves and broken red chilli . Saute . Add green chilli , ginger and onion .
Saute till onion turns transparent . Add green peas .( if fresh n tender add peas as such . else partially cook before and add )
Add water and bring to boil .
Meanwhile dilute besan in 1/4 cup water such that there are no lumps .Set aside.
When water starts boiling add mashed potatoes and salt . Mix well . Cover and cook for 2-3 mins so that potatoes takes in flavor .
Finally add besan diluted in water . Mix well . Keep the flame on sim . Bring to boil . You will see that the gravy will thicken and attain a nice consistency . If required add some more water . Taste test .
Switch off. Squeeze lime and garnish with chopped coriander leaves .
Thats it .. hot potato saagu is ready to enjoy . Serve it with poori / rava idli / set dosa / chapathi.