Boil or steam potatoes. Peel and mash boiled potatoes by hand into coarse chunks.
Heat oil in a frying pan or kadhai. Add the urad dal, red chillis, cumin seeds, and mustard seeds. When they splutter, add the asafoetida and curry leaves.
Add green chillis and ginger and fry for 1 minute. Add the turmeric and potatoes and 1 cup water. Add salt to taste and red chilli powder if using. Increase heat and boil for 5 minutes.
Mix besan in some water and add this paste to the potatoes. Cook a few minutes till the sabji thickens. Add in lime juice till you get the sourness you want. Garnish with chopped coriander leaves and serve.