Spread half a teaspoon of mustard over each slice of bread. Top two of the slices with half of the grated cheese followed by a slice of ham each, then finish with the remaining cheese on top. Sandwich together with the other two slices of bread.
Heat the butter in a large frying pan until foaming, then add the sandwiches and fry for 1-2 minutes on each side, or until golden-brown.
Transfer the sandwiches to a baking tray and place into the oven for 4-5 minutes, or until the cheese has melted.
Place the sandwiches on a serving plate, slice each sandwich in half and serve immediately.