Peel the potatoes, cut them lengthways into four segments per potato. Rinse in cold water to remove any excess starch, then place them in a pan with enough lightly-salted water to completely cover them. Bring the water to the boil and cook for just 8-10 minutes (they should remain slightly undercooked). Remove from the heat and drain off the water. Now cut into slices about ¼" (0.5cm) thick.
Meanwhile peel and slice the onion and fry gently in the fat for about 8 minutes. Add the sliced potatoes to the pan and continue cooking over a gentle heat, turning occasionally, until they become a light golden-brown. (If during this time, the onions start to brown, remove them from the pan with a slotted spoon and set aside until the potatoes are cooked, then add them back to the pan for a minute prior to serving). Sprinkle with a little freshly-chopped parsley and serve hot.