Temper: Heat a pan with oil, add cumin, mustard and curry leaves. When the spices splutter add hing, green chilies, ginger, potatoes and peas. Saute for 2 to 3 mins.
Cook the potatoes: Cover the potatoes with water and cook until fork tender. Meanwhile grind the fried gram to a fine powder and mix with water, turmeric and salt to make a thick slurry of pouring consistency.
Make the Sagu: When the potatoes are cooked, pour the fried gram slurry and stir well to prevent lumps. Simmer the gravy till it thickens. Turn off the heat
Add lemon juice and adjust salt to taste. Garnish potato sagu with coriander leaves. Serve with rava idli, poori, set dosa or sada dosa.