In a skillet over medium heat, melt 1 tablespoon of unsalted butter. Add the thinly sliced medium onion and a pinch of salt, cooking for about 10 minutes until golden brown and soft. Stir in 1 tablespoon of balsamic vinegar and cook for another 2-3 minutes. Remove onions from heat and set aside.

Using the same skillet, melt the remaining tablespoon of unsalted butter over medium heat. While the butter melts, slice your medium apple and pear. On two slices of sourdough bread, layer half of the shredded sharp cheddar cheese, followed by the caramelized onions, apple slices, and pear slices. Top with the remaining cheese and place the other bread slices on top to create a sandwich.

Carefully place one sandwich in the preheated skillet and grill on medium heat for about 4-5 minutes until the bread is golden brown and crispy, and cheese starts to melt. Flip the sandwich gently and continue grilling until the other side is golden and cheese is fully melted, about another 3-4 minutes.

Once both sides are grilled to perfection, remove from skillet and let cool slightly. Slice the sandwich in half, exposing the gooey goodness, and serve warm.