Peel and finely chop the onions, carrots and garlic, then trim and finely chop the celery. Deseed and finely chop the chilli. Pick and finely chop the rosemary.
Heat a couple of lugs of olive oil in a large saucepan over a medium heat, add the sage leaves and fry for 30 seconds, or until crisp, then remove to kitchen paper.
Place the pan of flavoured oil back on the heat, add the onion, carrot, garlic, celery, chilli, rosemary and a good pinch of sea salt and black pepper, then cook gently for 10 minutes, or until sweet and soft.
Meanwhile, halve, deseed and rough chop the squash. When the time's up, add the squash and stock to the pan. Bring to the boil and simmer for 30 minutes.
For the croutons, drizzle a little olive oil over the ciabatta, pat it in, then finely grate and press some grated Parmesan on to each side. Place in a dry, non-stick pan and fry until golden on both sides.
When the squash is tender, whiz the soup with a stick blender or pour it into a blender and pulse to a smooth purée – leave it slightly chunky, if you prefer.
Season to perfection with salt and pepper, then divide between bowls, placing 2 croutons on top of each.
Sprinkle with a few crispy sage leaves and drizzle with a swirl of quality extra virgin olive oil, then serve.