This silky soup is so much more than the sum of its parts. At our cafes, we use a well-scrubbed waxy yellow potato and leave the peel on, but peeled russets are a fine substitute. The starchiness of the potato determines the texture of the soup, so you may need to thin it with a bit more water or cream before serving.
Makes about 3 quarts
2 tablespoons butter
2 tablespoons olive oil
3 large leeks (about 1 pound)
1 small yellow onion
1 teaspoon kosher salt
2 ½ pounds potatoes (about 4 large)
4 cups water
4 ounces sour cream
3/4 cup heavy cream or half and half
Kosher salt, to taste
Freshly ground pepper, to taste
In a large heavy-bottomed soup pot, heat the oil and butter over medium. Clean and dice leeks and onion, sprinkle with salt and sauté until very soft and transparent. Scrub and dice potatoes then add to the pan with the water. Bring to a boil and simmer until potatoes are tender when pierced with a knife, about 15 minutes. Puree with an immersion blender or, working in batches, transfer soup to a blender jar, filling it no more than halfway full, and puree until smooth. Return soup to pot and whisk in sour cream and cream. Taste and season with salt and pepper, then thin with additional water if necessary. Gently warm the soup just until heated through. Serve immediately, with warm crusty bread.