Heat 2 tablespoons of the butter in a large, heavy saucepan. Add the onion and leek and saute slowly until they are tender but not brown.
Add the potato, cauliflower, anchovy, and water.
Bring to a simmer and cook, uncovered, for 1 hour.
Allow the soup to cool briefly, then puree it in a blender or food processor. You will probably have to do this in batches.
Return the soup to the saucepan, bring it to a simmer, and season it with salt and white pepper.
Just before serving time, heat the remaining butter in a small skillet. Saute the bread slices on both sides until they are golden and serve them alongside the soup.