3 Tbs. olive oil
1 tsp. fennel seed, coarsely crushed or ground in a spice grinder
1 medium fennel bulb, trimmed, cored, and cut into small dice (about 2 cups; save some fronds for optional garnish)
1 large leek (white and light-green parts only), halved lengthwise, rinsed well, and cut into small dice (about 1 cup)
1 Tbs. Pernod (optional)
3 cups lower-salt chicken broth
28-oz. can whole peeled plum tomatoes, drained and coarsely chopped (reserve the juice)
1 medium red or yellow potato, peeled and cut into medium dice
Kosher salt and freshly ground black pepper
Tip: This soup can be puréed 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 puréeing.
In a non-reactive 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 purée in a blender. If you purée the soup and it is too thick, add some of the reserved tomato juice. Garnish with chopped fennel fronds, if desired.