Gruyère Grilled Cheese Recipe

Ingredients

• 1 large yellow onion, thinly sliced
• 1 tablespoon olive oil
• 1 tablespoon butter
• 1 teaspoon brown sugar
• 1/4 teaspoon salt
• 1/4 teaspoon black pepper
• 2 slices sourdough bread
• 2 tablespoons unsalted butter, softened
• 4 ounces Gruyère cheese, shredded or thinly sliced
• 1 teaspoon Dijon mustard