Melt the butter in a large saucepan over a low/medium heat. Add the shallots and sweat for a minute. Crush the garlic and add to the pan, sweating for a minute more.
Finely chop the rosemary and add to the pan along with the cauliflower florets. Stir well to combine and cook for a minute or two until fragrant.
Add the stock and bring to a gentle boil. Reduce to a simmer for 6-7 minutes or until the cauliflower is tender.
Add the spinach and stir to combine until just wilted.
Transfer the soup to a blender or use a stick blender to process until smooth.
Divide between four bowls and drizzle each with a tablespoon of heavy cream to serve.