Chop the onion into approximately 1cm pieces and place in a large saucepan with the oil.
Season with salt and pepper and fry on a medium heat for 5 minutes stirring frequently.
Peel and grate the ginger and finely chop the chilli and garlic.
Add all to the pan and cook for another 5 minutes.
Turn down the heat and add the cumin, coriander and turmeric as well as the tomatoes and tomato puree.
Cook for 5 more minutes take care not to burn the spices.
Peel the sweet potato and cut into 3cm chunks.
Turn the heat back up to medium, add the sweet potato and water, mix well and simmer for 20 minutes or until the sweet potato has cooked through.
Serve over rice with chopped cashew nuts, fresh coriander and a spoonful of sweet potato.