Start by making the spice powder. Roast all the spice powder ingredients in a dry frying pan for 1-2 minutes until fragrant. Leave the mixture to cool, then grind to a fine powder and set aside.
Drain the soaked cashew nuts, reserving the liquid. Add the nuts to a blender, along with 3-4 tablespoons of the soaking liquid, and blitz to make a smooth paste. Remove and set aside. In the same blender, blitz the ginger and garlic with 2 tablespoons of water and grind to a smooth paste.
Heat the oil in a heavy-based saucepan over a medium heat. Add the onions and fry for 8-10 minutes, stirring, until they are light brown. Add the garlic and ginger paste and fry for a further 1 minute, stirring well to make sure it doesn’t stick to the bottom of the pan. Reduce the heat to low and add the chilli powder. Let it cook for 5 seconds, then add the cashew nut paste and ground spices and stir well for 1 minute.
Keeping the heat on low, add the Greek yoghurt, a little at a time, stirring well to ensure it doesn’t split. Fry for 5 minutes until the gravy begins to thicken and the oil begins to separate. Add the milk and stir well, making sure there are no lumps in the mixture. Add the mushrooms, sugar and salt and stir well. Increase the heat to high and bring to the boil, then reduce the heat to low, cover and simmer for 5 minutes.
Add the green peas and fresh coriander. Turn off the heat and keep warm until ready to serve. Serve with your choice of bread or rice.