Put the oil, butter, leeks, celery and onion in a large pot. Cover and cook about 8 minutes, stirring every minute or so, until the onion is translucent and the leeks have softened. Add the potatoes and water. Bring to a boil, lower the heat to medium, and cook until the potatoes are soft, about 12 to 15 minutes. Add the asparagus, milk and cream. Stir and cook, covered, at least 5 minutes longer. Season with salt and pepper, and sprinkle the chopped green onions on top of the soup just before serving. Wonderful soup!
Recipe courtesy of Curtis Aikens, Curtis Cooks with Heart and Soul, Hearst Books/William Morrow, 1995