Add a few tablespoons of finely grated gruyère or emmental cheese while puréeing to transform this into cauliflower cheese cream.
Put the cauliflower and onions in a large pan, then pour in enough milk to almost cover. Bring to a simmer and cook for 15-20 mins, or until the cauliflower is completely tender.
Using a slotted spoon, transfer the cauliflower and onion to a blender with a ladleful of milk, and blitz until smoothly puréed. Add more milk if you like, mixing to soften to your preferred consistency. Season with salt and pepper. (Can be made to this stage a day ahead. Keep chilled and reheat before serving.) Serve with a knob of butter melting on top.