Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the beef and onion and fry, stirring and breaking up the mince with a wooden spoon, until browned.
Stir in the garlic, spices, oregano, sultanas, tomatoes and stock. Add a little salt and pepper and bring to the boil, stirring. Spoon into the slow cooker pot, cover with overlapping slices of sweet potato, dot with butter and sprinkle with dried chillies and a little salt and pepper.
Cover with the lid and cook on low for 7–8 hours until the potato topping is tender. Lift the pot out of the housing using oven gloves and brown under a hot grill, if liked.