Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the onion and fry, stirring, for 5 minutes or until just beginning to turn golden.
Add the peppers, courgettes and garlic and fry for 2 minutes, then mix in the tomatoes and the wine or stock. Mix the cornflour to a smooth paste with a little water, then stir into the pan with the rosemary leaves and some seasoning. Bring to the boil, stirring.
Tip half the mixture into the slow cooker pot, add the unrolled belly pork and cover with the rest of the vegetable mixture. Cover with the lid and cook on high for 7–9 hours or until the pork is almost falling apart. If you like your sauces thick, ladle it out of the slow cooker pot into a saucepan and boil for 5 minutes to reduce down. Cut the pork into 4 pieces, then spoon into shallow dishes and serve with mashed potatoes and the tomato sauce.