Over a medium flame, heat three tablespoons of the oil in a large, shallow casserole dish for which you have a lid.
Tip in the chopped onion and cook, stirring, for 15 minutes, until softened. Add the garlic and rosemary, and cook for another five minutes.
Put a large frying pan on a medium-high heat, add a tablespoon of oil, then lay in the sausages and fry, turning often, for about five minutes, until nicely browned all over.
Add the squash to the onion pan, pour in the stock, cover and leave to simmer for five minutes.
Drain the butter beans into a sieve, rinse them under the cold tap, then stir into the sausage pan, cover and simmer for 20 minutes.
Put three tablespoons of oil in a large frying pan over a medium heat, tip in the breadcrumbs and toast for about five minutes, until golden and crisp. Season with salt and leave to cool.
Strip the cavolo nero leaves off the stalks, then tear them into bite-size pieces. Add these to the sausage pan, then put the lid back on and cook for five minutes, until the cabbage wilts.
Mash a quarter of the butter beans with a fork to thicken the sauce and turn it creamy, then grate in the parmesan and lemon zest, and squeeze in the juice of half the lemon. Season to taste.
Spoon the sausages and beans into bowls, top with the crisp breadcrumbs, and serve with a wedge of lemon if you like.
