Heat the oil or butter ghee in a pot, on medium heat. Add the mustard seeds and once it starts to crackle add the onion
Sauce the onion until golden brown. Add the curry leaf, chillies and ginger and garlic paste. Fry for a few seconds until the raw smell disappears
Add the chilli powder, coriander, cumin and turmeric. Fry for a minute on low heat
Add the potatoes and allow it to fry in the spices for at least 5 minutes. Add a couple of drops of water if required
Add the blended tomatoes and cook for at least 10 minutes. If the potatoes are slightly tender add the rinsed beans. Season with salt
If the potatoes take a long time to cook, add half the water and allow the potatoes to cook until tender and then add the beans. Add the remaining water and allow the curry to cook until the potatoes are soft and tender and the sauce/gravy is slightly thick.
Garnish with fresh coriander