The long, slow cooking of the onions gives this soup its rich caramelized-onion flavor and color.
Be sure to remove the bay leaf after simmering the soup. It is very dry and brittle and will not soften during cooking so it isn't edible.
Gruyère cheese is a rich, nutty, buttery-tasting form of Swiss, without the holes. It becomes smooth and creamy when melted.
Slowing cook the onions, stirring every 5 minutes to prevent burning, until deep golden brown.
Place ovenproof bowls or individual ceramic casseroles in a pan with shallow sides. Broil until cheese is melted and golden brown.