Peel the onions and cut them into halves, and then cut them into 1/8 inch thick slices. In a big, heavy pot heat the canola oil over medium heat. Add the onions and the salt. Stir so that the onions are covered with oil and then fry the onions on medium heat until they turn translucent, for about 20 minutes. Lower the heat to low and let the onions caramelize for about half an hour to an hour, scraping the bottom of the pan and mixing the onions with a wooden spoon. You should scrape the bottom of the pan and mix the onions quite often so that they do not burn, yet caramelize. At the end of the cooking the onions should have a deep amber color, be soft and sweet. Deglaze the onions with calvados and apple cider vinegar. Let it cook down and then add the apples, thyme, cayenne and the stock. Cook on low heat for about half an hour. Add freshly ground pepper and check for the saltiness of the soup and adjust if needed. Heat the oven to 350 degrees F. Cut the bread into slices, spread some butter on top and toast the bread in the oven until golden. Remove the toasted bread from the oven, spread some apple sauce on top and place brie slices over the apples. Ladle the soup into heat proof bowls, put a toast on top. Transfer the bowls into the oven and broil them until the cheese has melted. Serve hot from the oven.