This soup is quick and cheap and goes from fridge to table with a quick sprinkle of garnishes, which can be fancy or plain. The classic topping is snipped chives, but I like to serve it with a baked-potato toppings bar: grated or crumbled cheeses, crème fraîche, black pepper, sliced chiles, macerated tomatoes, diced avocado, picked crab or lobster, cooked baby shrimp, crumbled bacon, diced egg, etc. Serve the soup in wide bowls with large spoons for scooping up some garnish with every bite.
- Heat the fat in a large saucepan set over medium-low heat, add the leeks and give them a good stir. Sweat 8 minutes, reducing the heat as necessary so the leeks pick up no color.
- Add the potatoes, water, and salt and bring to a simmer. Cook until potatoes are tender, about 25 minutes. Let cool slightly then puree in batches. Stir in the half-and-half.
- Taste the soup: It should taste highly seasoned with salt as the cold mutes salinity in food. Adjust the seasoning, then chill the soup until very cold, at least 4 hours, and preferably overnight. Ladle into bowls, top with desired garnishes, and serve cold.