Wash tomatoes and cut into big chunks. Remove stalk of green chilly and wash. Place tomatoes and chilly in a mixture jar to make fine paste. Ginger can also be ground with these ingredients.
Whisk the curd thoroughly.
Wash sweet potato thoroughly, peel and place in water. Like wise peel all sweet potatoes. Take one sweet potato and cut in thick pieces (1 inch). Similarly, cut all sweet potatoes.
Preheat oil in a pressure cooker. Add cumin seeds and saute for while. After this add asafoetida, turmeric powder, coriander powder and saute for few minutes. Now add tomato-green chilly-ginger paste and red chilly powder. Stir constantly until oil starts separating from the mixture.
Once oil starts separating, add sweet potato chunks into it. Saute for 2 minutes. Add 1 to 1.5 cups of water in cooker. Cover the pressure cooker with lid until sabzi simmers once.
After sabzi simmers, add whisked curd into it and cook until it boils again. Now add salt and close the pressure cooker. Cook the sabzi until one whistle. Reduce the flame after one whistle and cook for another 3-4 minutes on low flame. Turn off the flame. Allow the steam to escape and add garam masala and green coriander in the sabzi. Mix well. Sabzi is ready, take it out in a bowl and serve hot with chapatti, parantha or rice and relish eating.
Gravy for sweet potato sabzi can be prepared as per our wish. Either use tomato or curd, or both as mentioned above.
Curd should be at room temperature, else it might coagulate.
After adding curd in sabzi, stir constantly until it simmers fro once. After it starts boiling, add salt else curd can coagulate.