Brush a medium sized non-stick pan with the olive oil or butter and sauté the onions over low heat for about 5 to 7 minutes, stirring often so onion won’t burn. Pour the beaten eggs into the pan. Cover for 4 minutes and let egg mixture cook over low heat. Flip egg mixture over and layer the cooked side of the omelette with the avocado, cheese and tomato. Cover the omelette again and cook for 3 minutes, or until cheese melts. Fold the omelette onto itself and season with salt & pepper. Cut omelette into 4 servings. Serve for breakfast, lunch or dinner and enjoy!