In a skillet over medium heat, melt 1 tablespoon of unsalted butter until it bubbles gently. Add thinly sliced medium onion with a pinch of salt and cook for about 10 minutes, stirring often, until the onions are golden brown and soft. Drizzle in a splash of balsamic vinegar and allow to cook for an additional 2-3 minutes, stirring continuously, until the vinegar thickens slightly. Remove the onions from the heat and set aside.

Using the same skillet, melt the remaining tablespoon of butter over medium-low heat. On two slices of sourdough bread, layer half of the shredded sharp cheddar cheese evenly. Next, add a generous portion of the caramelized onions, followed by thinly sliced medium pear and apple. Top off with the remaining cheese, and then cover with the other slices of bread.

Carefully place the prepared sandwich in the skillet and grill for about 4-5 minutes, checking for a beautiful golden-brown color on the underside. Flip the sandwich using a spatula and grill the other side for another 3-4 minutes until the cheese melts completely and the bread is crispy.

Allow the sandwiches to cool for a minute, then slice diagonally to reveal the gooey cheese with sweet apples and pears. Serve warm and consider pairing with a simple green salad or a light soup.