Heat the oil in a large frying pan and fry the onion for 8 mins until translucent. Add the garlic and fry for 1 min. Turn the heat to medium-high, add the aubergines and fry for 5 mins until golden. Stir in the courgettes and peppers and fry for 5 mins more until slightly soft. Add the tomato purée, fresh tomatoes, herbs, canned tomatoes, vinegar, sugar and 1 tsp salt and bring to the boil.

Transfer to the slow cooker and cook on low for 5-6 hours or until everything is soft and the sauce has thickened. Season, scatter over some extra basil, and serve with sourdough, if you like.