French Three Onion Soup

INGREDIENTS

• 1 tablespoon all-purpose flour
• Kosher salt and freshly ground black pepper
• 1/2 cup finely grated Gruyere (about 1 3/4 ounces)
• 1 teaspoon extra-virgin olive oil
• 1 tablespoon chopped fresh chives
• 6 shallots, thinly sliced, (1 3/4 cups)
• 1 1/2 pounds yellow onions, peeled, halved, and thinly sliced, (about 6 cups sliced)
• 1 tablespoon unsalted butter
• 1 fresh rosemary sprig
• 3 to 4 fresh thyme sprigs
• 2 teaspoons Worcestershire sauce
• 1 leek, white and light green part thinly sliced, washed well, and dark green top reserved
• 2 cups water
• 1/4 cup dry sherry
• 4 cups low-sodium beef broth
• 8 (1/4-inch thick) slices whole-wheat baguette (2 ounces), toasted