Heat the slow cooker if necessary. Heat the oil in a large frying pan and fry the onion until it starts to brown, then add the garlic and cook for 1 min. Add the vinegar and sugar and bubble for a minute. Stir in the beans and passata and season with black pepper. Tip everything into the slow cooker.
Cook on Low for 5 hours. If the sauce seems a little thin turn the heat to High and cook for a few more minutes. Stir through the coriander.