Cooking School: French Onion Soup

Ingredients

3 tablespoons unsalted butter

4 large onions (about 2 1/2 pounds total), halved and thinly sliced

1 teaspoon sugar

2 tablespoons all-purpose flour

2 cans (14.5 ounces each) beef broth (not condensed)

1/4 cup sherry or red wine

6 1/2-inch thick slices French bread, cut on a slight diagonal

1/4 pound Gruyere or Swiss cheese