Preparation:
In a large, heavy saucepan melt butter over moderate heat; add the onions and cook, uncovered carefully stirring until golden - 10 to 15 minutes. Blend in sugar and flour; cook, stirring, 3 minutes longer. Add broth, thyme, bay leaf and pepper; raise heat to moderately high and bring to a boil, stirring constantly - about 6 minutes. Adjust heat so mixture bubbles gnetly and simmer, partly covered, for 30 minutes. Remove and discard bay leaf. Heat broiler. Ladle soup into 4 flameproof bowls. Float 2 pieces of bread in each bowl and distribute cheeses evenly over top. Place bowls in broiler, 4 to 6 inches from heat. Broil until golden - about 2 minutes.
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