If you have never tried fennel soup, you are really missing out on something unique. It's like a leek and potato soup with a bit more interest and a great silky texture that just begs for a roaring fire on a wintry day.
Toss the fennel and onions with 1 tablespoon of the oil on a large heavy baking sheet, coating the vegetables. Roast in the oven, stirring occasionally, for 25 minutes, or until the fennel and onions are tender. Set aside.
Heat the remaining 1 tablespoon oil in a large heavy saucepan over medium heat. Add the garlic and sauté for 1 minute. Add the leeks and sauté for 5 minutes, or until they have softened slightly.
Add the stock, potatoes, and roasted fennel mixture, and bring to a simmer. Continue simmering for 18 minutes, or until the potatoes are very tender. Remove the soup from the heat and let it cool slightly.
Working in batches, puree the soup in a blender until smooth. Strain the soup into a clean large saucepan. Add the cream to the soup and bring to a simmer over medium-high heat, stirring occasionally.
Season with salt and pepper to taste. Ladle the soup into bowls, garnish with the chives, and serve.