Spray a pan with low calorie cooking spray, then sauté the onion until it starts to soften.
Chop up the cauliflower and broccoli into florets, then add these to the pan.
Pour in the water, then add the stock cubes.
Bring to the boil, stir, then cover and simmer for 25-30 minutes.
Remove from the heat, then blitz until smooth using a stick blender.
Check the seasoning and add a little salt and pepper if necessary. Serve on its own, or sprinkled with a little Danish Blue or Stilton.