Toss avocado, sour cream, lemon juice and Cheese in a bowl. Set aside. Melt butter in a frying pan and evenly coat the pan with it. When butter is foaming, let it subside. Whip pepper and salt through beaten eggs. Pour in beaten eggs and tilt pan so they spread evenly. When omelet starts to set, loosen edges and carefully lift them, tilting pan so the uncooked eggs run to the bottom. Spoon avocado filling onto one half of the omelet and fold over the other half. Cook over a low heat for about 3 minutes on each side. Serve topped with the chili sauce if desired (see recipe below).