French Three Onion Soup

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