The key to good onion soup is to use well-flavored yellow onions. For variation, this soup can also be served covered with a layer of Parmesan cheese in place of Gruyère.
In a large saucepan, melt the butter and cook the onions very gently until they are a deep golden brown, 15-20 minutes. Add the stock, salt and pepper, simmer for 10-15 minutes and taste.
Heat the oven to 300°F/150°C and bake the bread until dry and lightly browned, 15-20 minutes. Reheat the soup on top of the stove. Set two or three slices of bread in individu