Melt the butter in a large pot over medium heat. Add the leeks and onion, and cook for 10 minutes over medium-low heat. Add the potatoes, then the stock, and bring to a boil. Lower the heat, and simmer for 35 minutes, partly covered.

Put through a vegetable mill (and then, if you want a very thin soup, through a chinois). Return the liquid to the pot, and season with salt and pepper to taste. Bring back to a boil, and whisk in the sour cream.

When the soup has cooled, refrigerate for at least 6 hours. Serve cold, preferably in coffee cups, and sprinkled with dill.


Note: A version made by an Italian friend in Provence replaces leeks with zucchini (reduce cooking time by 10 minutes) and uses mascarpone instead of sour cream and basil instead of dill. Assolutamente Deliciosa.