Peel the potatoes and slice them into medallions (extra thin slices). Dice the Onion finely and place aside. In a large frying pan, fill to approx ? inch full of olive oil and heat. Stir in the potatoes and onions until just barely covered with the olive oil (sealing them). Fry for five minutes, constantly stirring so as not to burn. When the potatoes are tender, place a plate or lid over the frying pan and drain off the excess oil (putting aside in a heat proof container.