Potato, Leek, and Fennel Gratin
- Kosher salt and freshly ground black pepper
- 2 large fennel bulbs (about 1 ½ pounds), bulbs halved, cored, and thinly sliced crosswise, about 6 heaping cups; trimmings of tops and fronds coarsely chopped, about 2 heaping cups; divided
- 2 large or 4 medium leeks (about 2 pounds), whites and light green parts thinly sliced (about 6 heaping cups), trimmings of dark green tops coarsely chopped into 1-inch pieces (about 4 heaping cups); divided
- 2 (lightly packed and heaping) cups pea shoots, or one (10-ounce) bag frozen peas
- ½ cup plus 2 tablespoons olive oil, divided
- 1 cup (lightly packed) chopped cilantro (leaves and stems)
- 4 pounds Yukon Gold potatoes, peeled and sliced 1/4-inch thick
- 6 cloves garlic, peeled and roughly chopped
- 2 tablespoons toasted, crushed coriander seeds
- 1 pint (12 ounces) cherry tomatoes, halved
- ½ bunch cilantro, including stems, roughly chopped
- 1 to 2 tablespoons lemon juice
Preheat oven to 400°F.
Bring a large pot of water to a boil. Salt generously and add fennel slices. Let cook, 2 minutes. Add leek slices, and if using frozen peas, add peas. Cook 1 minute more. Using a large spider or sieve, strain contents of pot and transfer to an ice bath. Let cool and drain. Pat dry.
Return water to a boil and add leek and fennel trimmings. Boil until very tender, about 7-9 minutes. Drain, reserving 2 cups blanching water, and transfer trimmings to a blender or food processor. Add pea shoots if not using frozen peas, ¼ cup olive oil, cilantro, 1 teaspoon salt, ¼ teaspoon pepper, and 1 ¼ cups reserved blanching water. Blend until smooth, adding up to ½ cup additional water until a loose, chunky puree is formed. Season to taste with additional salt and pepper.
Coat a 5-quart baking dish or Dutch oven with 2 tablespoons olive oil. Scatter one-fourth of blanched leeks and fennel slices on the bottom. Begin shingling slices of potato so they are standing up, seasoning with salt and pepper and alternating adding blanched leeks and fennel from time to time in between slices. Scatter with garlic and coriander seeds and pour puree on top, using a spatula to work the puree in between potato slices.
Drizzle top with 3 tablespoons olive oil. Cover and transfer to oven. Bake until potatoes are tender, about 45 minutes to 1 hour. Remove from oven, uncover, and preheat broiler. Broil until the top darkens, about 6-8 minutes more.
Let gratin rest, at least 15 minutes.
Meanwhile, combine cherry tomatoes and cilantro in a medium bowl. Toss to coat with remaining 2 tablespoon olive oil and lemon juice. Season to taste with salt and pepper.
Scoop gratin onto plates, topped with tomato-cilantro salad. Serve.