Steps

1. Heat the olive oil in a large pot over medium-low flame. Add the onions and sauté slowly until they turn a deep caramel brown. This will take about 30 minutes. Take care to keep the heat low enough that the onions do not burn.
2. Stir in the chicken and beef broth and simmer for around 15 minutes.
3. Preheat broiler to high. Add the sherry, thyme, salt and pepper to the soup and simmer another 5 minutes.
4. Ladle soup into ovenproof bowls or crocks. Lay a piece of bread over the top of each bowl of soup. Then place a slice of cheese over each piece of bread.
5. Place bowls under the broiler for 3-5 minutes, or until the cheese is bubbly and starting to brown. Serve immediately.

Notes

For a milder, chewier cheese, substitute provolone for the Gruyère.