4 slices firm white bread
2 tablespoons softened unsalted butter
4 ounces thinly sliced Swiss cheese (4 slices), preferably Gruyère or other imported cheese
2 ounces thinly sliced boiled or baked ham (2 slices)
Dijon mustard
Heat a cast iron grill pan or griddle over medium heat until hot but not smoking. Butter one side of each piece of bread. Lay the slices on a cutting board with the buttered sides down. Divide the cheese evenly among the four slices (you may have to fold the slices in half to fit the bread). Put ham on two slices, spread each with a generous teaspoon of Dijon mustard, and then put the remaining two cheese-covered slices on top.
Put the sandwiches in the grill pan and cook for 1 minute, pressing lightly with a spatula; then turn and cook the other side for 1 minute. Turn the sandwich back over, cook for 1 minute or until the toast is golden brown; turn again and cook until both sides are golden, rotating them about a quarter turn from where the original grill marks are to finish cooking and create those attractive cross-hatch grill marks. Remove the sandwiches from the grill, cut them into halves, and serve.