Guinness & Onion Soup

I immediately flagged this soup recipe from Pennies on a Platter when I read it two weeks ago.  When I planned it for our menu last week, I checked out the original recipe on Food Network.  I noticed that the original recipe called for sherry vinegar, which we had, so I chose to use that in this soup.  We also made a few other variations.  We made it meatless by omitting beef broth and substituting mushroom broth.  Also, instead of toasting bread with cheese on it, we chose to go the traditional 'French Onion Soup' route and float a piece of bread in the soup, and cover the whole bowl with cheese before toasting.  This soup was quite delicious, especially with a nice tall glass of Guinness to go with it!

2 tablespoons Olive Oil
5 Garlic Cloves, minced
8 cups Yellow Onions, thinly sliced (about 2 large onions)
1 tablespoon Fresh Thyme Leaves, chopped
1/4 cup Sherry Vinegar
1 1/2 cups Guinness Draught (not Stout)
6 cups Beef Stock (we used mushroom stock)
1 Bay Leaf
Bread, cut into 6 1/2 inch slices
1/2 pound Irish Cheddar, thinly sliced (we used Provolone)

Heat the olive oil in a dutch oven over high heat.  Stir in the garlic and cook until just fragrant.  Add the onions, season with salt, and cook for about 5 minutes, stirring often.  Reduce the heat to medium-low and saute the onions for about 15-20 minutes, stirring occasionally, until they are a deep amber color.  Add the thyme, vinegar, and beer.  Cook until the liquid is reduced by half, then add the stock and bay leaf.  Bring to a simmer and cook for about 20 more minutes.  Discard the bay leaf and ladle the soup into individual broiler-safe bowls.  Place a piece of bread in each bowl and top the bowl with cheese.  Place the bowls under the broiler until the cheese is brown and bubbly.