Preheat the oven to 170C/150C fan/gas 3. Peel the potatoes and finely slice. Lay out on a baking tray (nonstick or lined with baking parchment), drizzle with 1 tbsp of the olive oil, season and bake for 15 minutes.
Select a large ovenproof frying pan about 20cm diameter and 4cm deep. Heat the remaining oil in the pan, add the onion and cook over a medium heat for 5 minutes, stirring occasionally.
Remove the potatoes from the oven and layer with the onions in the frying pan. Lightly beat the eggs with the chives and pour over the potatoes and onion. Season with salt and freshly ground black pepper and mix.
Transfer the pan to the oven and continue to cook for 20-30 minutes. Serve hot or at room temperature.