Place the potatoes in a large pan of salted water and bring to the boil. Reduce the heat and simmer for 10 minutes, until softened.
Drain and allow to cool a little, then cut into 5mm-thick slices.
Heat the olive oil in a large, heavy-based pan over a high heat. Add the sliced potatoes and onions and cook for 6 to 7 minutes, until the potatoes are slightly
browned and the onions have softened.
Season with salt and pepper and transfer to a