We've updated the classic soup with whole-grain barley, whose hearty chew is a tasty addition. Pearl barley may be quick-cooking, but it's not whole grain; choose whole-grain barley here, sometimes labeled "hulled barley" and often found in the bulk foods section.
6 cups water
1 cup hulled barley
2 tablespoons canola oil
1 1/2 pounds yellow onions, peeled and vertically sliced
1 pound sweet onions, peeled and vertically sliced
3/4 teaspoon kosher salt, divided
6 garlic cloves, thinly sliced
1/4 cup dry sherry
6 cups unsalted beef stock
1/2 teaspoon freshly ground black pepper
3 thyme sprigs
1 bay leaf
1 1/2 tablespoons fresh thyme leaves
3 ounces French bread baguette, cut into 18 thin slices
Bring 6 cups water and barley to a boil in a large saucepan; cover, reduce heat, and simmer 1 hour or until done. Drain.
While barley cooks, heat oil in a Dutch oven over medium-high heat. Add onions; cook 5 minutes, stirring frequently. Add 1/4 teaspoon salt and garlic; reduce heat to medium-low, and cook 45 minutes or until onions are caramelized and very tender, stirring occasionally.
Increase heat to medium-high. Add sherry; cook 2 minutes or until liquid almost evaporates. Stir in stock, pepper, thyme sprigs, and bay leaf; bring to a boil. Reduce heat, and simmer 30 minutes. Discard thyme sprigs and bay leaf. Stir in barley, thyme leaves, and remaining 1/2 teaspoon salt.
Preheat broiler to high.
Arrange bread in a single layer on a baking sheet; broil 30 seconds on each side or until toasted. Ladle 1 1/3 cups soup into each of 6 ovenproof bowls. Arrange 3 toast pieces in each bowl; divide cheeses evenly over toasts. Place bowls on baking sheet. Broil 2 minutes or until cheese melts and begins to brown.