Add the lemon juice to about a litre of cold water in a mixing bowl (the lemon juice prevents the potatoes from browning). Add the peeled potatoes. Butter a 20 cm ovenproof dish with duck fat or butter. Adjust the oven rack to the middle position. Preheat the oven to 200°C. Slice the potatoes thinly, do not put them back in the water again but place the slices in a dry mixing bowl. Season with Potatoe Spice, oil the sliced potatoes and toss with your hands to coat all round. Starting from the edge of a round ovenproof dish, arrange the potato slices in a spiral shape, moving inwards. Drizzle over the melted duck fat or butter. Place in oven and bake for 1 hour or until golden brown and cooked through.
To keep the potatoes upright in the beginning, skewer a few slices on a toothpick to support the rest while you pack them out.