Heat oil in a large soup pot. Add onions and sauté over medium-low heat for about 20 minutes, stirring occasionally until caramelized. They probably will stick to the bottom of the pot, so use a wooden spatula to scrape off the bits, keeping them in, which is going to flavor the soup tremendously.
Once onions are dark brown, add thyme. Stir for 30 seconds, then add stock. Bring to a boil, then reduce heat to a simmer and cook for approximately 15 minutes.