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Heat oven to 200C/180C fan/gas 6. Rest each potato on a large serving spoon and cut widthways at 3mm intervals – cutting the potato on a spoon stops you slicing all the way through. Repeat with all the potatoes. Roughly grind the peppercorns with a pestle and mortar.
Melt the butter and oil in a large roasting tin. Once sizzling, add the potatoes and pared lemon zest. Toss well so that all the potatoes get coated in the fat, then season well with salt and three quarters of the freshly ground black pepper. Roast on the bottom shelf of the oven for 1 hr-1 hr 15 mins until golden and tender. Discard the lemon zest and scatter over the remaining pepper to serve.