French Onion Soup Gratinee | America's Test Kitchen

Ingredients

• 2 tablespoons unsalted butter
• 5 medium red onions (about 3 pounds), sliced thin
• Table salt
• 6 cups low-sodium chicken broth (canned)
• 1 3/4 cups low-sodium beef broth (canned)
• 1/4 cup dry red wine
• 2 sprigs fresh parsley leaves
• 1 sprig fresh thyme
• 1 bay leaf
• 1 tablespoon balsamic vinegar
• Ground black pepper
• 1 baguette , cut on the bias into 3/4-inch slices (2 slices per serving)
• 4 1/2 ounces Swiss cheese , sliced 1/16-inch thick
• 1 1/2 ounces grated Asiago cheese (about 3/4 cup)