Butternut squash risotto with balsamic vinegar

1 small butternut squash or 1 Kabocha squash (about 450g)

¾ cup sunflower oil

¼ onion, diced

salt

700g Carnaroli or Vialone Nano rice

2.4l stock

110g Parmesan, grated

1 tbsp unsalted butter

aged balsamic vinegar, for drizzling

For the broth:

2 celery stalks, halved

2 carrots, halved

1 onion, halved

3l water