Heat your oven to 200°C/Fan 180°C/Gas 6. Peel the sweet potatoes and cut them lengthways into quarters. Pick the sage leaves off their stalks and lay them on top of the sweet potatoes. Loosely wrap the strips of pancetta around the sweet potato wedges and sage. Arrange in a snug roasting tin or ovenproof dish. Sprinkle over a little salt and pepper, and drizzle 1 tbsp oil over the top. Roast in the oven for 35 mins.
Meanwhile, peel and finely slice the onion. Peel and crush or finely grate the garlic clove. Strip the rosemary leaves from their stalks and finely chop the leaves. Slice the chestnut mushrooms.
Pour 1 tbsp oil into a large pan and bring to a medium heat. Add the onion, garlic and rosemary. Add a pinch of salt and pepper and fry for 5 mins, stirring occasionally, till softened. Tumble in the mushrooms and continue to cook for a further 5 mins, till the veg is glossy and caramelised.
Open the tin of borlotti beans and pour both the beans and their juices into the mushrooms, and stir well to combine. Bring to the boil, then reduce to a simmer. Trim the dry ends off the cavolo nero and roughly shred the leaves. Add to the pan and stir. Pop a lid on the pan and simmer for 10 mins, stirring constantly, till tender.
When the sweet potatoes have roasted for 35 mins, tear up the mozzarella and scatter on top of the sweet potatoes. Return to the oven for 5 mins, till the mozzarella has melted.
Scoop the bean and mushroom ragù into bowls and serve topped with the pancetta and mozzarella sweet potatoes.