• 6 medium leeks
• ¼ cup vegetable oil
• 4 large boiling potatoes, peeled and diced
• ½ pound fresh mushrooms, brushed clean and sliced (optional)
• 8 cups chicken stock or water
• 1 egg
• 3 tablespoons fresh lemon juice
• Salt and freshly ground black pepper

Trim off the root end and all but about 2 inches of the green tops from the leeks and cut in half lengthwise. Cut the halves crosswise into pieces about ¾ inch wide. Place the pieces in a sink full of cold water, swish them around to loosen any dirt, remove with a slotted spoon, and drain well in a colander.

Warm the vegetable oil in a large saucepan over medium heat. Add the leeks, potatoes, and the mushrooms, if using, and sauté for about 5 minutes, stirring occasionally. Raise the heat to high, add the stock or water, and bring to a boil. Reduce the heat to low, cover, and simmer until the vegetables are soft, about 40 minutes.

In a bowl, beat together the egg and lemon juice until quite frothy. Whisk in a little of the hot soup to temper the egg, then gradually stir the egg into the soup over very low heat. Cook very gently for about 1 minute, but do not boil. Season with salt and pepper. Ladle into bowls and serve at once.