Gently heat the oil in a large pan. Fry the sausages all over until brown, then remove and set aside. Add the onion to the pan and fry for 5-10 mins until golden and soft, then add the celery, garlic and rosemary, and cook for 2 mins more. Pour over the stock and bring to the boil.

Reduce the heat and simmer for 20 mins, then add the sausages back to the pan along with the butter beans. Simmer for 8 mins, then stir in the butter, parsley and lemon zest. Ladle into bowls and serve piping hot with lots of chunky bread.