Cook the unpeeled potatoes in boiling salted water for 15-20 minutes until tender when tested with the point of a knife. Allow to cool slightly, then cut into chunky slices.
Heat the oil in a heavy-based non-stick frying pan big enough to take all the ingredients, about 28cm. Add the onions and cook for 4-5 minutes until soft and beginning to brown.
Meanwhile whisk together the eggs and chives. Season generously with salt and freshly ground black pepper then add the cheese and whisk again. Heat the grill to moderately hot. Add the potato slices to the onions and then pour over the egg mixture. Cook over a low heat until the edges are beginning to firm up and the frittata is lightly set. This may take up to 10 minutes.
Place the pan under the grill. Don’t place the pan too close to the heat or it will burn on top before the centre is cooked. Cook for 2-3 minutes until the eggs are set and the top is a lovely golden colour. Serve hot or cold. This would be delicious with a mixed leaf and tomato salad.