Tip everything except the cream and croutons into the slow cooker along with 500ml hot water. Cook on high for 3 hrs or low for 5 hrs.

Remove the rosemary. Blend the soup either using a hand blender, which you can do in the slow cooker, or in a jug blender until smooth. If you like a thinner soup, you can add up to 250ml more hot water. Will keep chilled for three days. Spoon into bowls, drizzle over a little cream and scatter with croutons, if using.