In a large pot over medium-low heat melt the butter. Add the onions with a pinch of salt and sugar then sweat until softened, this will take around 10-15 minutes.
Add the cauliflower and vegetable stock, turn up the heat and bring to the boil. Now turn down the heat to a simmer and cook until the cauliflower has softened, this will take around 15 minutes.
Add the cream, nutmeg and lemon, bring back to a simmer. Add the mixture into a food processor and blitz until smooth.
Add the soup back to the pot and if necessary reduce further until you have your desired consistency. Check and adjust seasoning if needed.
Serve fresh from the pot with a sprinkle of chives and croutons.