Heat the oil in a large, heavy-based frying pan and cook the onion over a medium heat, stirring occasionally, for 2 minutes. Add the garlic purée and rosemary and cook, stirring constantly, for 30 seconds.
Add the beans and tomato sauce and bring to the boil. Reduce the heat, cover and simmer for 6–7 minutes until piping hot.
Serve with fresh, torn wholemeal bread for mopping up the juices.