Leek and Potato Soup with Shoestring Potatoes and Fried Herbs

For a beautiful, almost bone-white soup, don't let the leeks and onion take on any colour as they soften.

Soup:
½ cup (1 stick) butter
6 large leeks, white and pale-green parts only, halved lengthwise, thinly sliced crosswise
1 small onion, thinly sliced
5 cloves garlic, thinly sliced
sea salt
2 cups whole milk, divided
1 cup heavy cream, divided
1 cup plain whole-milk yogurt, divided
black pepper, freshly ground

Potatoes and Assembly:
1 russet potato, peeled, cut into thin matchsticks
vegetable oil, for frying (~ 4 cups)
¼ cup fresh flat-leaf parsley leaves
1 sprig rosemary
sea salt
flaky sea salt, such as Maldon

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.

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°F (180°C/160°C fan, Gas Mark 4). Working in batches and returning oil to 350°F 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 sea salt. Serve soup topped with a nest of potatoes and herbs and seasoned with sea salt.

Make-Ahead: Soup can be made 2 days ahead. Cover and chill. Potatoes and herbs can be fried 2 hours ahead. Store uncovered at room temperature.