If you prefer, you can substitute 4 medium onions, preferably Vidalia, in place of the leeks.
Prep time: About 20 Minutes Cooking time: About 35 Minutes Trim root ends and green parts off the leeks and discard (or save greens for stock). Cut the whites of the leeks down the middle lengthwise and clean thoroughly under cold running water. Chop the leeks into 1-inch pieces. In a large soup pot, heat the oil and butter over moderately low heat. Add the leeks and cook, stirring frequently, for 15 minutes, or until softened but not brown. If the leeks begin to burn, reduce the heat to low. Add the potatoes and salt and pepper and cook for 1 minute. Raise the heat to high and add the broth. Bring to a boil, reduce heat to a low simmer, and let cook, partially covered, for 15 minutes, or until the potatoes are tender--but not falling apart--and the broth is flavorful. Working in batches, blend the potato-broth mixture in a food processor or a blender until smooth. Taste for seasoning; add more salt or pepper if needed. For a creamier, thicker soup, add some or all of the cream and blend until smooth. (You can refrigerate the soup for 1 to 2 days or freeze it for 3 to 6 months at this point.) Serve in individual bowls with a sprinkling of chives, a swirl of cream, and croutons, if desired. How kids can help: Help clean the leeks; press the food-processor buttons; add garnishes to the soup.