Boil the potatoes and remove the skin. You could either cook the potatoes in a pressure cooker or in the microwave. If cooking in the pressure cooker, make sure that the potatoes do not become too mushy. I usually remove the skin of the potatoes and then cook it in the microwave. Cook it until they are knife tender.
Let it cool down a bit and then grate the potatoes in a large holed grater. I use my box grater for this.
In a large pan, heat the oil and then add the mustard seeds. Once the seeds splutter, add the urad dal, channa dal, grated ginger, green chilies, curry leaves and asafetida. Fry them for about 30 seconds.
Now add the grated potatoes and salt to taste. Mix it well and let it cook for about 2 minutes.
Remove the curry/poriyal from flame and then add the lemon juice. Mix well and serve!