Mushroom Grilled Cheese with Balsamic Caramelized Onions

Bahan-bahan (2 porsi)

• 1 tablespoon olive oil
• 1 medium onion, (sliced thinly)
• 1 pinch kosher salt
• 2 tablespoons balsamic vinegar
• 1 tablespoon olive oil
• One 8-ounce package sliced mushrooms
• ¼ teaspoon kosher salt
• freshly ground black pepper
• ¼ cup white wine
• ¼ cup (½ stick) salted butter, (softened)
• 2 tablespoons grated Parmesan cheese
• 2 teaspoons chopped fresh rosemary
• 4 slices bread
• 4 ounces jack, havarti or white cheddar cheese, (sliced)