Preheat the oven to 190C / Gas 5. Lightly butter an ovenproof dish. Slice the potatoes lengthways to a thickness of about 3 – 4mm. Scatter the potatoes, pancetta and sage leaves into the dish, seasoning with salt and pepper as you go. Stir the lemon juice into the stock and pour over the potatoes. Dot a little more butter over the top and roast for an hour or so until the top is golden and potatoes are melting and tender. Coarsely grate the cheese. Spoon the one-pot onto plates and top each serving with a handful of cheese. Serve swiftly.