Remove the root end and any damaged outer leaves from the leek, but leave the remainder of the leaves intact. Split the leek in half lengthwise, and cut it into 1/2-inch pieces. Clean the leek by immersing the pieces in a bowl filled with cold water. Lift the pieces from the water and place them in a sieve to drain. Heat the oil in a pot. When hot, add the leek and sliced onion, and cook over medium heat for about 5 minutes, until they soften and begin to brown lightly. Add the stock, potatoes, salt and pepper, and bring to the boil. Boil for 30 to 40 minutes, until the potatoes are tender. Strain off most of the cooking juices and reserve them. Add the solids with a little of the juices to the bowl of a food processor, and process briefly, just until pureed. (If too much liquid is added to the processor bowl, the mixture will become too foamy.) Stir the puree into the reserved juices. You should have about 7 cups. The hot soup can be served immediately, either plain or with croutons. FOR COLD SOUP: Cool the soup and stir in the milk, chives and Tabasco sauce. Serve cold. Serves 6. *If you cannot find leeks, add additional onions or scallions; the soup will be very good, although it won't have the unique taste that leeks give to soup. If using leeks or scallions, be sure to make use of the green as well as the white part of the vegetables, after trimming them first, of course, to eliminate any damaged or wilted sections. The green leaves lend color as well as taste and texture to the soup. Recipe from "Cuisine Economique" by Jacques Pepin.