In a non-stick skillet, heat the oil. Saute onions till golden. Add the vegetables, except the bell peppers/ capsicum. Sprinkle a little salt and stir-fry till almost done. With your spatula, spread the vegetables so they cover the base of the pan evenly. Sprinkle the chopped bell peppers/ capsicum evenly over this and pour the egg mixture in. Turn the heat down. Cover the pan and let the omelet cook slowly. Make sure the underside does not burn.