Clean champignons and cut them into pieces. Fry on half the butter with additional oil. Dice the cheese into 1 cm cubes. Separate egg whites from yolks. Mix the yolks with a tablespoon of water and flour, add salt. Whip the egg whites with a pinch of salt. Carefully combine ingredients by mixing from the bottom up.
Heat oil with butter on a pan, pour the eggs and spread evenly. Lever up the edges and pour the excess mixture there. Reduce heat. When bubbles appear on the surface of the omelette, place a strip of champignons and cheese along the middle. Fold the edges of the omelette to close the filling inside. Turn it over and dry the omelette to become fried inside.
