Peel the broccoli stem.
Cut the stem into thick slices.
Separate the broccoli tops into small florets. (There should be about 3 cups of broccoli altogether.) Melt the butter in a large saucepan.
Add the broccoli, leeks, fennel, and water.
Slowly steam-saute the vegetables over low heat, stirring until they begin to soften, about 20 minutes.
Add 1 1/2 cups of the stock, the milk, coriander, and nutmeg.
Bring just to a boil, cover, and simmer over low heat for about 15 minutes or until the vegetables are quite tender.
Process the soup in a blender on low speed or in a food processor until smooth.
Add more stock, if desired, to thin the soup slightly.
Heat through, if necessary, before placing in a warmed soup tureen.
Float lemon slices topped with fennel sprigs on the soup, if desired.