In a medium pot, place the butter and onions over medium heat. Cook the onions for about 20 minutes or until golden brown. Add the wine and cook for about 7 minutes more.
Add the beef and chicken stock and bring to a boil. Then reduce the heat to medium-low and add the salt and pepper. Simmer for about 25 minutes.
Ladle soup in oven proof bowls or ramekins, top with the toast and add the cheese on top of the bread. Broil until golden, for about 1 minute.