Golden Grilled Cheese Sandwich with Sweet Onions and Gruyère - Cook with Carla

Ingredients

• 60 millilitres beef broth, dry sherry, or water
• 1 teaspoon fresh thyme leaves, chopped
• 1/4 teaspoon freshly ground black pepper
• 1/2 teaspoon fine salt
• 1 teaspoon granulated sugar
• 3 medium yellow onions, peeled, halved, and thinly sliced
• 2 tablespoons olive oil
• 2 tablespoons unsalted butter
• 115 grams sharp white cheddar cheese, shredded
• 170 grams Gruyère cheese, shredded
• 4 slices hearty sourdough bread