½poundgrated Gruyere cheeseor use Swiss or Provolone cheese
Melt butter in a wide bottomed pot set over medium heat then add sliced onions, sugar, and salt.
Slowly caramelize the onions until they are a deep brown color, stirring occasionally to ensure they caramelize evenly. For the best flavor, this cannot be rushed. Expect this to take 30 minutes or more.
Once the onions have almost finished caramelizing, add the minced garlic and cook for two more minutes.
Reduce to low heat, sprinkle in the flour, and cook for 3 more minutes, stirring to prevent the flour from burning. Then add the beef stock and scrape the bottom of the pan with a wooden spoon to loosen up any burnt on bits.
Add Worcestershire sauce, bay leaf, and black pepper, and bring the soup to a simmer. Let simmer uncovered for 45 minutes.
Ladle soup into oven-safe bowls and place toasted baguette slices and grated cheese on top of the soup. Place under a broiler until the cheese has melted then serve immediately.
If you simmer your soup too long it can thicken too much or become too salty. This can be fixed by dribbling a little water into the soup to thin it out.