Ingredients:

• 6 (1-inch thick) slices of a baguette
• 1 1/4 teaspoons Dijon mustard
• 1 cup grated Gruyere cheese
• Freshly ground black pepper
• 1/4 cup unsalted butter or vegetable oil
• 2 1/2 to 3 pounds yellow or red onions, halved, peeled, and sliced 1/4-inch thick
• 1 tablespoon brown sugar
• 1 1/2 teaspoons kosher salt
• 1/4 cup dry sherry or vermouth
• 4 cups low sodium beef broth
• 1 teaspoon Worcestershire sauce
• 1/4 teaspoon ground black pepper
• 1/4 teaspoon dried thyme leaves or 1 teaspoon fresh thyme leaves
• 1 bay leaf