Gourmet Grilled Cheese: Elevate Your sandwich recipes!

Ingredients

• 2 large yellow onions, thinly sliced (about 400g)
• 2 tablespoons olive oil (30ml)
• 1 tablespoon butter (15g)
• 1/2 teaspoon salt (2.5g)
• 1/4 teaspoon black pepper (1.25g)
• 1 tablespoon balsamic vinegar (15ml)
• 1 teaspoon brown sugar (5g) (optional)
• 4 slices sourdough bread (about 1 cm thick)
• 4 tablespoons unsalted butter, softened (60g), divided
• 6 ounces (170g) Gruyere cheese, shredded or thinly sliced
• 2 tablespoons Dijon mustard (30ml)