Trim 2 fennel bulbs, reserving the fronds, and quarter the bulbs.
Place in a large saucepan with butter, cream, and milk.
Cover, bring to a boil, then reduce heat and simmer on low heat until the fennel is very tender. Remove with a slotted spoon, leaving liquid in the pan.
Blend fennel with parmesan and some chopped fronds to a smooth paste.
Gradually add the hot cream liquid until a thick but pourable sauce is formed.
Season with salt and white pepper.
Refrigerate until required.