Melt butter in a large pot and gently cook the onion for 10 minutes or until soft. Add the cauliflower, potato, milk, stock and some seasoning, then bring to the boil.
Simmer for 25 minutes or until the vegetables are completely tender. Blend until smooth, return to the pot and check seasoning.
To serve, reheat the soup if necessary and ladle into warmed bowls (or large mugs). Top with the cheese and some freshly ground black pepper. For the full effect, stir through the cheese just before eating. Serve with bread, if you like.