French Onion and Apple Grilled Cheese – Fast Comfort Food

Ingredients

• 4 slices of sourdough bread
• 2 tablespoons unsalted butter, divided
• 1 medium yellow onion, thinly sliced
• 1 teaspoon sugar
• 1/2 teaspoon salt
• 1 tart apple (such as Granny Smith), thinly sliced
• 1 cup grated Gruyère cheese
• 1/2 cup shredded mozzarella cheese
• 1 teaspoon fresh thyme leaves (optional)
• Ground black pepper to taste