This soup takes a lot of time and attention (and a tireless stirring arm). End-to-end it takes me about 2 hours and 45 minutes. I think it's worth it, though, and the soup reheats well for about a week after.
The flavor of the onions becomes rich, deep, and caramel-y. The other key to making this soup really special is the time under the broiler. My thick, sturdy supper club bowls withstand the heat but get very hot to the touch. We have to eat off hot pads.
For cheese I like to use Gruyère or Asiago.