Peel the sweet potatoes and cut into 2 cm (1 inch) thick rounds or pieces.
Soak the raisins in a bowl of very hot water for 5 minutes.
Heat the oil and butter in a large, heavy-based casserole and soften the onion over a gentle heat. Add the spices, salt and pepper and stir for 5 minutes. Moisten with the water. Once it bubbles, add the sweet potato and cook for 10 minutes.
Add the drained raisins, then the cinnamon, sugar and honey. Lower the heat and leave to simmer for 10 minutes, until the sauce is thick and rich.