1. Place leeks in a large bowl of cold water; stir, and let stand 5 minutes to let dirt and sand settle to the bottom. You may need to repeat several times, changing the water each time. Once they are free of grit, transfer to paper towels to drain.

2. Melt butter in a large saucepan over medium-low heat. Add onion and fennel seeds along with the salt and pepper. Cook, stirring occasionally, until onion is softened and translucent, about 10 minutes. Add garlic; cook until fragrant but not brown, about 2 minutes.

3. Add leeks and fennel to pan, and cook until vegetables are tender, about 10 minutes. Add chicken stock and the water, and bring to a boil; reduce heat, and simmer 20 minutes more.

4. Remove from heat, and let cool slightly. Working in several batches, transfer soup to the jar of a blender, being careful not to fill more than halfway and to cover lid with a kitchen towel, as hot liquid will expand when blended. Puree until smooth. Transfer to an airtight container, and place in refrigerator to chill completely, at least 4 hours or overnight.

5. To serve, stir in Pernod, if using, and season with salt and pepper. Ladle into soup bowls, and garnish with chervil.