Preheat the oven to 200ºC, fan 180ºC, gas 6. Rub the potatoes with the olive oil, then bake for 50 minutes, until the skin is crisp and the potato is soft.
Remove the potatoes from the oven, cut in half, spoon most of the potato into a bowl and reserve. Put the skins on a baking tray, then cook in the oven for a further 10 minutes.
Meanwhile heat a large frying pan to medium. Place the pancetta in the pan and cook for 3-4 minutes, until crisp and golden. You may need to do this in batches. Add the chopped rosemary and rosemary sprig to the pan for the final 1-2 minutes of cooking time.
Take 200g of the reserved potato (see Cook’s tip), season with freshly ground black pepper and mix with half of the spring onions. Break the pancetta into pieces and mix half with the remaining spring onions and the red cabbage.
Remove the potato skins from the oven and fill with the potato mixture, top with the cabbage mixture, then scatter over the remaining pancetta. Serve with the soured cream and reserved rosemary sprigs.
Cook’s tip: Use any leftover potato to make bubble and squeak.