In a skillet, melt 1 tablespoon of butter over medium heat. Add the sliced onions and cook until caramelized, about 10 minutes.
Spread remaining butter on one side of each slice of bread. Place one slice in the skillet, butter side down. Layer with pear, apple, caramelized onions, and cheese. Top with another slice of bread, butter side up.
Cook until golden brown, about 3-4 minutes on each side. Remove from skillet and drizzle with balsamic vinegar. Season with salt and pepper to taste.
Notes
This sandwich can be customized with different cheeses or additional vegetables.