Heat the butter in a frying pan. Over a medium heat, fry the slices of bread for approx. 3 mins. on each side. Place two slices on a baking tray, sprinkle with cheese.
Cook for approx. 5 mins. in the centre of an oven preheated to 180°C. Remove from the oven, top with the roast beef, cover with the remaining slices of bread, serve immediately.