Melt the butter in a soup pot and sauté the onions until very soft, at least 20 minutes. Add the Cognac and the wine one minute. Add the stock. Simmer, covered, 45 minutes. Check the seasonings. Ladle into oven-proof soup bowls and set on a baking sheet. Top each with a croûte. Sprinkle the top generously with cheese. Broil until the top is lightly brown, about 2 minutes.