Place the whole butternut in the slow cooker and leave on low for 5-6 hours. (I had mine on low overnight and it was fine).
Once tender, cut open, discard the seeds, then transfer the flesh to a soup maker (this is mine) or a pan, leaving the skin behind.
Now add everything else to the soup maker or pan. Set the soup maker to smooth and serve. If using a pan, simmer for 20-22 minutes until everything is tender and blitz until smooth.