Heat olive oil in a skillet over medium heat. Add the sliced onion and a pinch of salt. Cook, stirring occasionally, for about 10-15 minutes until the onions are soft and golden brown. This slow cooking allows the natural sugars to caramelize, enhancing the flavor.

Once the onions are caramelized, add balsamic vinegar and stir for 1-2 minutes. Remove from heat and set aside.

Spread softened butter on one side of each slice of sourdough bread. This will create a golden, crispy crust when grilled.

On the unbuttered side of four slices, layer the aged cheddar, Gruyère, and mozzarella cheeses evenly. Top with a generous spoonful of caramelized onions, then place the remaining slices of bread on top, buttered side facing out.

Heat a skillet over medium heat. Once hot, carefully place the assembled sandwiches in the skillet. Grill for about 4-5 minutes on one side until golden brown.

Using a spatula, flip the sandwiches carefully and grill the other side for another 4-5 minutes until the cheese is melted and the bread is golden brown.

Drizzle a little truffle oil over the sandwiches during the last minute of grilling for an extra gourmet touch.

Once grilled to perfection, remove the sandwiches from the skillet. Let them rest for a minute before slicing in half. This allows the melted cheese to set slightly, making it easier to eat.

Serve warm, optionally with a side of tomato soup or a simple salad for a complete meal.