In a large, heavy pot melt the butter in the olive oil over medium-high heat. Add the onions and garlic and reduce the heat to medium. Cook, stirring continuously, for about 20 minutes or until the onions are nice and brown. Season with pepper.

Add the flour, stirring it into the mixture so it cooks for a minute or two. Add the mustard and stir to incorporate.

Add the wine and the bay leaves and cook for about 5 minutes. Add the stock a little at the time, and simmer very slowly for at least 30 minutes. If the stock is evaporating too quickly, or if you have an extra guest at the last moment, add a cup or two of extra stock or water.

Just before serving, preheat the oven to broil. Mound about two-thirds of the cheese on the toast slices and place them on a baking sheet. Broil, watching very carefully, until the cheese is melted and lightly browned.

Ladle the soup into warm soup bowls, add a tablespoon of cheese to each bowl for extra richness, and then float the cheese toast on top.