Best French Onion Soup

INGREDIENTS

• :
• 3 tablespoons unsalted butter, cut into 3 pieces
• 6 large yellow onions (about 4 pounds), halved and cut into ¼-inch thick slices
• Table salt
• 2 cups water, plus extra for deglazing
• ½ cup dry sherry
• 4 cups chicken broth
• 2 cups beef broth
• 6 sprigs fresh thyme, tied with kitchen twine
• 1 bay leaf
• Ground black pepper
• 1 small baguette, cut into ½-inch slices
• 8 ounces shredded Gruyere cheese (about 2½ cups)