This could not be simpler. On one slice of the buttered bread, spread half the grated Gruyere, then cover that with the slices of ham, folding them if need be to fit the size of the bread. Now sprinkle the rest of the Gruyere on top of the ham, season, then press the other slice of bread on top of that and press it down very firmly. You can at this stage cut off the crusts, but I think they add extra crunchiness.

Now brush half the melted butter on the top side of the sandwich, sprinkle it with half the Parmesan and press it in. Now transfer the sandwich to the grill pan and grill it for about 2 minutes, 2 inches (5 cm) from the heat. When it's golden brown, turn it over, brush the other side with the remaining melted butter, sprinkle the rest of the Parmesan all over and grill for another 2 minutes. Then remove it from the grill, cut it into quarters and eat it while it's still crunchy.