Add onion, broccoli and cauliflower and simmer until the florets are tender.
Liquidise in a food processor or with a hand held blender.
Return to the pan and add the milk. Gently bring to a steady simmer, then cover and cook for 10mins.
Stir in the cheese and simmer for a few more mins, occasionaly stirring until the cheese has completely melted and the soup has thickened slightly. DON'T allow the soup to boil at this point or the cheese will go stringy.