Onion Soup with Parmesan Croutons

Cut off roots and leaf ends from leeks. Discard any tough outer leaves. Cut each leek lengthwise in half, then crosswise into 1/4-inch-wide slices. Place leeks in large bowl of cold water; with hand, swish leeks around to remove any sand. Transfer leeks to colander. Repeat process, changing water several times, until all sand is removed. Drain well. In 8-quart saucepot, melt margarine or butter and olive oil over medium-high heat. Add leeks, onions, shallots, and thyme, and cook, covered, until tender and deep golden brown, 40 to 45 minutes, stirring occasionally. Remove cover and increase heat to high. Add brandy and cook 1 minute, stirring and scraping bottom of saucepot. Add chicken broth, salt, pepper, and 4 cups water; heat to boiling. Reduce heat to low; cover and simmer 20 minutes. Meanwhile, preheat oven to 450 degrees F. Place bread slices in 15 1/2-inch by 10 1/2-inch jelly-roll pan or on large cookie sheet; bake 3 minutes. Turn slices over; sprinkle tops with Parmesan cheese and bake 5 minutes longer or until toasted. Top each serving of soup with a Parmesan crouton.