Coat a large deep pot with olive oil. Add the onions and sprinkle generously with salt. Bring the pan to a medium heat. Stir the onions to coat with oil. Cover the pot and sweat for 15 to 20 minutes.
Once the onions have gotten very soft and let off a lot of water, remove the lid and let the onions cook looooow and sloooooooooow. That's right low and slow to really develop their rich brown color and sweet flavor. There is no shortcut or rush for this, DO NOT TRY! Caramelized onions take a long time, about an hour, accept it and move on.
When the onions are very brown and have gotten considerably smaller, add the sherry and cook until the sherry has reduced by half. Add the bay leaves, thyme bundle and chicken stock. Bring to a boil and reduce heat and simmer for at least 1 hour. TASTE for seasoning, you probably will need to season with salt. If the stock reduces too much add water to keep the liquid level up- this IS a soup after all. When done, the soup should be thick and very full flavored. TASTE! You may need to resason, caramelized onions are very sweet and require a fair amount of salt.
Toast or grill the baguette slices. Fill ovenproof crocks 3/4 of the way with the onion soup, float 2 toasts in each bowl and top with grated Gruyere. Place the crocks under the broiler to melt the cheese and make it brown and bubbly. Top with chopped chives.