Melt the butter in a 3-quart saucepan over medium heat. 

Add the chopped fennel and onion and cook over medium-low heat until vegetables are soft - about 20 minutes. 

Add salt, chile and pepper, and cook one minute more.

Add the tomatoes and all the juices form the can, stir well and bring to a boil. 

Reduce heat and cook a simmer, uncovered, for 30 minutes.  Set aside to let cool slightly.

Purée the soup in two batches in a blender, adding 1/2 cup of the vegetable stock to each batch. 

Put the soup through the finest disk of a food mill into a clean pan and add the pastis, stirring to blend. 

Thoroughly wash the feathery fennel fronds and reserve 8 small stems for garnish, if desired. 

Chop remaining fronds and stir in 1/4 cup (packed) to the soup.  Bring to a simmer and serve.