Peel the onion and chop it roughly. In a pot with a thick bottom (I love my dutch oven), melt ½ stick of butter over medium heat. Add onion and fry for 5 minutes while stirring. The onion should be lightly golden.
Reduce heat to low, add all-purpose flour, cook for 3 minutes, stirring frequently.
Get cauliflower broth on hand. Start adding the broth to the roux in small batches while whisking properly to avoid lumps. Once you’ve added about a half of the broth, you can add the rest at once and mix it in.
Add the cauliflower florets, don’t forget to save those ⅓ cauliflower chunks which you’ve put in cold water before! Bring to a boil, reduce heat, take the lid off the pot and allow the soup to simmer uncovered for 20 minutes.
Remove the soup from the stove. Pour in the heavy cream and stir it. Add ground pepper, a pinch or two of freshly ground nutmeg, salt to your liking. Do not cook anymore!
Puree cauliflower in the soup with an immersion blender properly. Add the remaining butter and let it melt in a soup. Add the ⅓ of the cauliflower you’ve set aside earlier, let it heat up in a hot soup, and serve.
As the final step, sprinkle the served soup with finely chopped green herbs (spring onion or chive are some good choices).