How to Make It
Cook sliced onions in hot oil in a Dutch oven over medium-high, stirring occasionally, 20 to 25 minutes or until onions are golden brown. Add garlic, and cook 1 minute. Stir in 8 cups water, bouillon granules, pepper, and thyme. Add sherry, if desired. Bring to a boil. Cover, reduce heat to low, and simmer, stirring occasionally, 20 minutes.
Ladle soup into oven-safe serving bowls, and place on a baking sheet.
Cut baguette diagonally into 8 slices. Stir together Swiss cheese, basil, and mayonnaise. Spread mixture on 1 side of bread slices. Top soup with bread slices, cheese side up.
Broil 5 inches from heat 2 1/2 to 3 minutes or until lightly browned. Serve immediately.