Potato Leek Soup

For 4 servings, use 500 g potatoes, 250 g leek, 100 g onion, 30 g butter, 900 ml chicken stock, 120 ml heavy cream, and 1 teaspoon salt. Split the leek lengthwise and rinse carefully between the layers where dirt can remain, then shake off the excess water. Peel the potatoes and cut them into small, even cubes. Thinly slice the leek and onion to a similar thickness so they soften at the same pace. Melt the butter in a pot over low heat. Add the leek and onion and cook for about 6 minutes, stirring often, until the vegetables look glossy and feel soft while retaining their pale color. Add the potatoes and chicken stock and bring the pot to a boil over medium heat. Reduce the heat to low and simmer for about 20 minutes, stopping when a potato cube crushes easily against the side of the pot. Turn off the heat briefly and blend with an immersion blender, reaching the bottom and edges until no potato lumps remain. Stir in the heavy cream and warm the soup over low heat for 3 minutes without a hard boil. Season with the salt. Serve the soup hot, or chill it thoroughly and present it cold in the style of vichyssoise.