cook them in really hot oil (not olive) in a wok or frying pan for about 1 min and no more than 1.5 mins, all you need is about 3-4 table spoon fulls of oil, when it starts to smoke throw the rings in and keep them moving for the time above.
As Webbo says, or blanch the rings quickly in boiling water. Don't cook them straight from the fridge, but take them out and let them warm to room temperature (15 - 20 mins) before cooking.