If time permits, prepare the sandwich 1 to 2 days before cooking. The flavours of the truffle will infuse the cheese over time.
Place one layer of the gruyere cheese on two slices of the bread. Carefully place a layer of shaved truffle (or truffle oil) on top of the gruyere cheese. Place a second layer of gruyere cheese on top of the truffle. Sandwich together, with the two remaining slices of bread. If time permits, tightly wrap in cling wrap and refrigerate for 1 to 2 days before cooking. The flavours of the truffle will infuse the cheese over time.
Prepare the barbecue for direct cooking over medium heat (180 to 230°C) and preheat with a Weber large frying pan or hotplate for 10 minutes.
Spread butter on the outer sides of the bread.
Once the barbecue has preheated, sprinkle two tablespoons of the parmesan cheese into the pan or hotplate, approximately the size of the bread. Immediately place the sandwiches on top of the parmesan cheese. Cook for 4 to 5 minutes over direct medium heat or until the outside is golden. Lift one sandwich at a time, sprinkle another tablespoon of the parmesan cheese into the pan or hotplate, place the sandwich (uncooked side down) onto the parmesan. Repeat with the other sandwich. Continue to cook the toasties for a further 4 to 5 minutes, or until the bread is golden and the cheese has melted.