Heat butter in a heavy-based saucepan.
Add the onion and sauté until translucent.
Add nutmeg and fry for another 2 minutes.
Add the butternut and mix through to cover with spice.
Sprinkle the cake flour over and mix through.
Gradually add 500 ml of the chicken stock to the butternut mixture while stirring.
Reduce heat, cover and allow to simmer for 20 minutes or until butternut is soft.
Allow to cool slightly.
Transfer to a food processor or use a stick blender and blend until smooth.
Return mixture to saucepan.
Add remaining chicken stock and milk.
Bring to the boil.
Reduce heat and allow to simmer for 3 minutes.
Stir in cheese and cream and season to taste.
Allow to simmer for 3 minutes or until heated through.
Serve with a dollop of sour cream and croutons.