Heat half the oil in a large saucepan, add the onion, garlic, ginger, spices and salt and pepper and cook over a low heat, stirring occasionally, for 10 minutes until the onion is softened.
Add the tomatoes and measurement water to the pan and bring to the boil, then reduce the heat, cover and simmer for 15 minutes. Add the potatoes and chickpeas, cover and cook for 20 minutes.
Meanwhile, heat the remaining oil in a frying pan, add the mushrooms and cook over a medium heat, stirring frequently, for 3–4 minutes until browned.
Add the mushrooms to the curry with the cashew nuts and fresh coriander and cook for a further 10 minutes. Stir in the yogurt and heat through without boiling. Serve with rice.