Traditional French onion soup uses beef stock. I prefer a roasted veal stock or roasted chicken stock. The beef is very strong and I find it overwhelms rather than complements the sweetness of the caramelized onions.
Combine the onions, stock, cognac, thyme, bay leaves and pepper in a large saucepan and . bring to a boil. Lower the heat to a gentle simmer, just a few little bubbles rising to the surface, and cook for 30 minutes.
Don’t forget to remove the thyme sprigs and bay leaves!
Ladle the soup into the bowls and top each with some cheesy crostini.