Cut the cauliflower florets from the stalk and thinly slice the stalk.Melt the butter in a pan over a moderate heat and add the onion, cooking it for 8-10 minutes until soft and translucent.
Add the garlic, bay leaves and sliced cauliflower stalk, and cook for 4-5 minutes more to soften. Add the stock, bring up to the boil and season well to taste with salt and pepper.
Add the cauliflower florets, reduce the heat to simmer and cook with a lid on for about 20-25 minutes, stirring from time to time, until the cauliflower is tender.
Add the milk, remove the bay leaves and blend until completely smooth- like liquid velvet. Check the seasoning, adding more salt and freshly ground black pepper as necessary. Warm through and stir through the Parmesan.
To make the sage and walnut butter to garnish, add the butter to a small pan over a moderate heat, add the walnuts and sage leaves, and cook until brown sediments begin to appear in the bottom of the pan.
Keep stirring often, you want the sediments to turn toffee-brown, the sage leaves to crisp and the walnuts to begin just to colour a little.Remove the pan from the heat and serve the butter, sage and walnuts spooned over the soup.