In a nonreactive 4-quart saucepan, heat the oil over medium-low heat. Add the fennel seed and cook until fragrant and lightly brown, about 3 minutes. Add the fennel bulb, leek, and Pernod (if using) and cook over low heat, stirring occasionally, until the vegetables are soft, about 10 minutes.

Add the broth, tomatoes, and potato. Bring to a boil over medium heat. Reduce the heat to low, cover, and simmer until the potatoes are cooked through, 30 to 40 minutes. Season to taste with salt and pepper. Serve chunky or puree in a blender. If you puree the soup and it is too thick, add some of the reserved tomato juice. Garnish with chopped fennel fronds, if desired.

This soup can be pureed in the blender for a velvety version of the original. Be sure to work in small batches and crack the blender lid slightly (or remove the center cap from the lid). Steam can build up once you start blending, and if the lid is on tight or the blender is overfilled, it will spray hot soup all over you and your kitchen. For protection, cover the top with a dishtowel while pureeing.