Peel the potatoes, cut them into large chunks and put them in a saucepan of cold salted water.
Bring to the boil and simmer for 15 mins until just cooked (do not overcook or the potatoes will absorb too much water).
Meanwhile, heat 1⁄2 tbsp of the olive oil in a small pan. Fry the pancetta in the same pan until the fat has melted and the pancetta is crisp.
Drain the potatoes and allow to steam-dry. Once cool enough to handle, chop the potatoes into 1cm dice and mix in a bowl with the pancetta, Gruyère, parsley, beaten eggs and chives. Season generously with black pepper.
Heat 1 tbsp of the oil in a large non-stick frying pan. Gently tip in the potato mixture and press down with the back of a large spoon.
Fry over a medium heat for 15 mins until the bottom is golden brown.
Tip the hash brown onto a plate, then slide back into the pan, cooked-side up.
Cook for another 15 mins until golden and hot. Serve with roasted tomatoes, if you like.