In a skillet over medium heat, melt 1 tablespoon of unsalted butter. Add thinly sliced onions and a pinch of salt, sauté for about 10 minutes until golden brown. Stir in balsamic vinegar, cooking for an additional 2-3 minutes until caramelized.

In the same skillet, add the remaining tablespoon of unsalted butter and melt over medium heat for grilling.

On two slices of sourdough bread, layer half of the sharp cheddar cheese, add caramelized onions, followed by thinly sliced pear and apple. Top with remaining cheese and cover with another slice of bread.

Grill the assembled sandwich in the skillet for about 4-5 minutes until golden brown. Flip and grill the other side for an additional 4-5 minutes until cheese is melted.

Once golden brown, remove the sandwich and let cool for a minute. Slice in half and serve warm.