Soak the cashews in 100ml of boiling water for 10 minutes. Meanwhile, crush the fennel seeds as finely as you can with a pestle and mortar.
Wash and scrub the potatoes – no need to peel. Cut them into chunks of the same size. Heat 2 tbsp of the oil in a lidded frying pan and fry the potatoes for around 8 mins, until they brown a bit, stirring every now and again. Remove the potatoes to a plate.
Transfer the cashews along with their soaking liquid to a blender and pulse to a fine paste.
Put the rest of the oil into the frying pan over a medium heat. When hot, fry the onion for 10 mins, or until soft and golden. Add the garlic and ginger, cook for 2 mins, then add the tomatoes. Cook for around 5-8 mins, until the sauce thickens and reduces.
Then add the crushed fennel seeds, garam masala, chilli powder and salt. Stir to mix, then add the cashew paste. Cook for a couple of minutes, then add around 300ml of water and stir.
Put the potatoes back in the pan, cover with the lid. Cook on a low heat for a further 10-15 mins, until really tender the whole way through. You can check by sliding a knife through: if there’s no resistance, they’re done.
