Melt the buttter in a heavy bottomed pan and then add the spring onions (and celery if using) and saute for 2 minutes.
Add the broccoli and stock, bring to the boil, cover the pot with a lid and cook for 15 minutes.
Remove the pot from the heat and add the cheese. Once the cheese has melted add the cream as well as the pepper and nutmeg.
Blend in a food processor until smooth.