Warm a heavy-based pan over low–medium heat. Add the butter and melt gently without allowing it to brown. Add the diced onion with the salt and sweat for 10 minutes until completely soft and translucent, stirring regularly to prevent colouring — any caramelisation will darken and sweeten the final purée.

Add the chopped cauliflower. Continue to sweat gently for 20–25 minutes, stirring occasionally. The cauliflower should soften fully and release moisture but remain pale; do not allow it to catch on the base of the pan.

Once tender, add enough heavy cream to lightly coat and surround the vegetables. Cook over medium heat and reduce until the cream thickens and just binds the cauliflower — it should move from loose and creamy to lightly glazed and cohesive, not wet or soupy.

Transfer immediately to a blender and puree until completely smooth, adding a small stream of olive oil during blending to enhance gloss and silkiness and the ground white pepper. Adjust consistency if needed with a splash of milk.