Trim away roots and dark green parts of leeks. Slice remaining white and light green parts in half lengthwise. Cut crosswise into ¼-inch-thick half-moons. Place in a salad spinner and wash thoroughly.

Heat olive oil and butter over medium heat in a large, heavy-bottomed pot. Add leeks and garlic and sprinkle with 1 teaspoon salt. Cook, stirring occasionally, until leeks are softened but not browned, about 10 minutes.

While leeks are cooking, fill large bowl halfway with cold water. Peel potatoes, placing each in bowl of water immediately after peeling to prevent browning.

Cut each potato in half lengthwise and slice into ½-inch-thick half-moons. Drain potato slices and add to pot along with stock, remaining salt, and pepper.

Raise heat to high and bring soup to a boil. Reduce to a simmer, cover, and cook until potatoes are very soft, about 20 minutes.

Puree soup with an immersion blender or in batches in standing blender. I like to use my Vitamix for a super-silky, luxurious texture.

Serve garnished with chopped chives, dill, and flaky sea salt.