Cut the potatoes into medium-sized pieces (halved or quarters), place them in a pan of water and bring to the boil. Turn down to a simmer, cook for 10 minutes until tender, then drain and set aside to cool.
Heat the oil in a frying pan over a medium heat and, once it’s hot, add the mustard and cumin seeds . As the seeds start to move about, add the onions and cook, stirring, for about 8 minutes, until they begin to brown.
Add the garlic, ginger, red or green chillies, cashew nuts and mushrooms and stir regularly, for another two minutes. Add the potatoes in a single layer and season with salt and pepper, turn up the heat to high and fry for about five minutes, stirring once, until the potatoes brown slightly.
Stir in the tamarind paste, pour in the coconut milk, and stir regularly, until all the coconut milk has evaporated and turned to a glaze.
