In a large soup pot, heat the olive oil over medium heat. Add the fennel and onion.
Saute until the vegetables are soft, about 5 minutes.
Stir in the potatoes, chicken broth, milk and lemon juice.
Cover, reduce heat and simmer until the potatoes are tender, about 15 minutes.
In a blender or food processor, puree the soup in batches until smooth.
Return the soup to the pot and heat until warmed through.
Ladle into individual bowls and garnish with toasted fennel seeds. Serve immediately.