In a skillet over medium heat, melt 1 tablespoon of unsalted butter. Add the thinly sliced onion and a pinch of salt, cooking for about 10 minutes while stirring occasionally until they turn a beautiful golden brown. Stir in 1 tablespoon of balsamic vinegar and cook for an additional 2-3 minutes. Set the onions aside.

Using the same skillet, melt the remaining tablespoon of butter over low heat. Layer half of your shredded sharp cheddar cheese on one slice of sourdough bread. Pile on the caramelized onions, followed by the sliced pears and apples. Finish with the remaining cheese and cover with another slice of bread.

Carefully place the assembled sandwich in the skillet over medium heat. Grill for about 4-5 minutes until the bottom slice turns golden brown and the cheese begins to melt. Gently flip the sandwich and grill the other side for another 4-5 minutes.

Once cooked to perfection, remove the sandwich from the skillet and let it cool slightly. Slice in half and serve warm.