In a medium skillet, melt 1 tablespoon of unsalted butter over medium heat. Add thinly sliced onions and a pinch of salt. Cook for about 10 minutes until soft and translucent.

Drizzle in 1 tablespoon of balsamic vinegar and cook for 2-3 more minutes until golden brown and caramelized.

In the same skillet, add more butter to melt. Lay two slices of sourdough bread, sprinkle half of the cheese on each slice, layer with caramelized onions, sliced apple and pear, and top with remaining cheese.

Place another slice of bread on top to assemble the sandwich. Grill for about 4-5 minutes, pressing gently with a spatula.

Flip sandwiches and grill for another 4-5 minutes until perfectly toasted.

Remove from skillet, slice diagonally, and serve warm.