6 cups sliced peeled onions
2 tablespoons all-purpose flour
6 cups beef broth
2 tablespoons white wine vinegar
1/2 teaspoon salt
1/2 teaspoon ground black pepper
1/2 pound French bread, sliced
6 ounces Swiss cheese, sliced
1/2 cup freshly grated Parmesan cheese
Preheat oven to 375 degrees. Melt butter in a heavy stockpot; sauté onions over medium-low heat until light brown (slightly caramelized), stirring occasionally, 8 to 10 minutes. Sprinkle flour over onions. Stir in beef broth, vinegar, salt, and pepper. Bring to a boil, reduce the heat, and simmer uncovered for 10 minutes.
Place French bread in oven-safe serving bowls. Ladle soup over bread and place a slice of Swiss cheese on top of each bowl. Sprinkle with Parmesan cheese. Bake until cheese has melted, about 5 minutes. Serve hot.
Makes 6 servings.