Take a wok, add oil and heat it.
Add cumin seeds, mustard seeds, slit green chillies, split black gram, bengal gram, saute for few seconds.
Add curry leaves and allow them to crackle.
Add the thickly sliced onions, mix well.
Add salt as per taste, turmeric powder, mix the masala well and cook for few seconds on low flame.
Add the mashed potatoes, mix everything well and cook for a minute or two on low flame.
Lastly, garnish with freshly chopped coriander leaves and give a mix.
Serve hot with dosa, poori or roti or chapati.