Start by heating a skillet on medium heat, large enough to hold the onions, and add in the butter.
Let the butter melt, then toss in the onions. Give them a good stir, reduce the heat to a medium-low, and continue to cook. Stir every few minutes or so and watch the temperature. You want these to go low and slow for the most part. This should take about 25 minutes or so, maybe longer. Be patient and enjoy the sounds and more importantly the smell.
Once the onions have softened to your liking, add in the stock. Give it a good stir, then add in the thyme, marsala wine, and shredded chicken. Stir again, and bring up the temperature to a medium heat. Cover and let cook for 30 minutes. Season with salt. Go with about a teaspoon, stir and wait a few minutes. Taste, and add more salt if you desire. This one needs a bit of salt.
When you are about to serve, take your bread and lightly toast it under the broiler. Once the bread begins to crisp, remove, add the cheese to the tops, and return to melt the cheese.
Ladle the soup into your bowl, and top with a big cheesy crostini. Dig in.