Heat the oil in a large, deep frying pan or wok, and add the diced sweet potato, aubergine and leek. Cook over a medium heat for 5 minutes, stirring regularly.
Add the mushrooms, garlic, ginger and spices, and cook for a few minutes more until the mushrooms are soft and the leeks are beginning to brown.
Add the coconut milk, spinach and vegetable stock. Bring to a gentle simmer, and leave to cook for around 10 more minutes, until the potatoes are tender. Season to taste.
Just before serving, add the cashew nuts to a dry pan over a medium-high heat, and toast them for a couple of minutes until golden brown. Keep an eye on them and shake the pan regularly, as they burn easily.
Serve the curry with basmati rice and / or naan bread, topped with fresh coriander.