Set a saute pan (ideally a nonstick one) that'`1s 8 to 10 inches across over medium heat. Add the butter and let it melt, tilting the pan this way and that so it coats the entire bottom of the pan and up the sides a bit.
Use a rubber spatula to lift one edge of the omelet and fold it over, so it forms a half moon. Lift the pan off the stove, tilt it, and slide the omelette onto a plate.
Feel free to add a few tablespoons of one of the following fillings along with the cheese: chopped ham, chopped spinach, crumbled bacon, or sliced sauteed mushrooms.