Melt the butter in a large heavy-bottom soup pot over medium heat. Add the onion. Cook, stirring occasionally, for 5 to 8 minutes, or until the onion begins to soften. Sprinkle the flour over the onion and cook, stirring constantly, for 1 to 2 minutes.
Gradually add the chicken broth and wine, whisking constantly. Bring to a boil, whisking frequently. Decrease the heat to low and add the bay leaf. Cover and simmer for 20 minutes.
Remove the bay leaf. Turn off the heat and stir in the heavy cream. Gradually add the cheese, one small handful at a time, stirring until each handful is melted before adding the next handful.
Stir in the Worcestershire sauce and ground pepper. If the soup is not hot, return to low heat until warmed through.
Top with the croutons and, if desired, cracked pepper.