From the Jan/Feb '08 issue of Cook's Illustrated

Soup:
Adjust the oven rack to the lowest position and heat to 400°F.  Generously spray the inside of a heavy-bottomed large (at least 7 quarts) Dutch oven with non-stick spray.  Place butter into the pot and add the onions and 1 tsp of salt.  Cook covered for 1 hour - the onions will be moist and slightly reduced in volume.  

Remove pot and stir the onions, scraping the bottom and sides of the pot.  Return the pot to the oven with the lid slightly ajar; continue to cook until very soft and golden brown: 1 1/2 - 1 3/4 hours longer.  Stirring and scrapping sides after 1 hour.

Carefully remove the pot from the oven and place over medium-high heat.  Use oven mitts to handle the pot.  Cook the onions, stirring frequently and scraping the bottoms and sides of the pot until the liquid evaporates and the onions brown, 15 - 20 minutes, reducing the heat to medium if the onions are browning too quickly.  
Continue to cook, stirring frequently until the pot bottom is coated with a dark crust, 6 - 8 minutes, adjusting heat as necessary.

Stir in 1/4 cup of water, scraping pot bottom to loosen the crust, cook until the water evaporates and the pot has formed another dark crust 6 - 8 minutes, until the onions are very dark brown.  Keep repeating this process 2 - 3 times more (it develops the flavor of the onions)

Stir in the sherry, cook stirring frequently until the sherry evaporates - about 5 minutes.

Stir in the broths, 2 cups water, thyme, bay leaf and 1/2 tsp salt, scraping up any final bits of browned crust on the bottom and sides of the pot.

Increase the temperature to high heat and bring to a simmer.  Reduce the heat to low, cover and simmer for 30 minutes.  Remove and discard the herbs, season with salt and pepper.

Brown the bread and float on top of the soup, cover it with the cheese and heat it under the broiler to melt - serve and enjoy the best French Onion Soup you will ever eat.