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.