½ cup (1 stick) unsalted butter
6 large leeks, white and pale-green parts only, halved lengthwise, thinly sliced crosswise
1 small onion, thinly sliced
5 garlic cloves, thinly sliced
2 cups whole milk, divided
1 cup heavy cream, divided
1 cup plain whole-milk yogurt, divided
Freshly ground black pepper
INGREDIENTS : Potatoes & Assembly
1 russet potato, peeled, cut into thin matchsticks
Vegetable oil (for frying; about 4 cups)
¼ cup fresh flat-leaf parsley leaves
1 sprig rosemary
Flakey sea salt (such as Maldon)
A deep-fry thermometer
INSTRUCTIONS – Soup
Melt butter in a large heavy pot over medium heat. Add leeks, onion, and garlic and season with salt. Cook, stirring often, until vegetables begin to soften, 5–7 minutes. Add 1 cup water and cook, stirring occasionally, until vegetables are very soft, 25–30 minutes. Let cool slightly, then transfer half of leek mixture to a blender and add half of milk, cream, and yogurt. Purée until very smooth; pour into a large bowl. Repeat with remaining leek mixture, milk, cream, and yogurt.
Return soup to pot and heat over medium, adding water by ¼-cupfuls, until soup is thick enough to hold potatoes without sinking but thin enough that a spoon dragged across the top doesn’t leave a trail. Season with salt and pepper.
INSTRUCTIONS – Potatoes & Assembly
Rinse potato in a colander under cold water until water runs clear; pat completely dry. Fit a medium saucepan with thermometer; pour in oil to measure 2”. Heat over medium-high until thermometer registers 350°. Working in batches and returning oil to 350° between batches, add potato by the handful and fry, turning occasionally, until golden and crisp, about 2 minutes. Using a spider or slotted spoon, transfer potato to paper towels to drain. Fry parsley and rosemary until parsley is translucent, about 15 seconds. Add to potato and let cool; season with kosher salt. Serve soup topped with a nest of potatoes and herbs and seasoned with sea salt.