1. Heat the butter in a large pot and gently fry the onions for 5 minutes, or until soft and golden.
2. Break the cauliflower into florets and thinly slice the stalks. Cut the potatoes into thin slices.
3. Add the vegetables to the pot together with the stock and milk, and bring to the boil.
4. As soon as the pot boils, turn the heat right down and simmer gently, stirring occasionally, for 25 minutes, or until the potato slices are very tender.
5. Let the soup cool for 15 minutes, then liquidise to a fine, smooth texture using a stick blender or food processor. Add the mayonnaise and nutmeg and blend again for a few seconds. Season to taste with salt and pepper.
6. To make the croutons, cut the bread into cubes and fry in oil over a medium heat until crisp and golden. Drain on a paper towel, then dust lightly with paprika.
7. Gently reheat the soup and serve hot with the paprika croutons.