Spray a large saucepan with low-calorie cooking spray and set on a medium heat. Fry the onion for 5 minutes until it starts to soften.
Add the cauliflower and broccoli to the pan, pour over the boiling water and crumble in the stock cubes.
Bring to the boil, then lower the heat and simmer for 25-30 minutes.
Remove from the heat, then blitz until smooth using a stick blender or food processor.
Check the seasoning and add a little salt and pepper, if necessary. Crumble over the Danish Blue or Stilton, if using, and serve.