Heat the olive oil in a casserole and gently fry the celery, carrots, onion, mushrooms, leek, and garlic for a few minutes until they start to soften, stirring. Then add the sausages and cook for a few minutes more.
Add the tomatoes, oregano and tomato puree and simmer for 5 minutes.
Add the beans and simmer for a further 10 minutes until the vegetables are soft and the sausages are fully heated through.
I sometimes add some cabbage, if I've got a little bit that needs using up.