Heat oven to 220C/200C fan/gas 7. Peel the potatoes, trim to rough rectangles, then use a mandolin or knife to slice as thinly as possible. Fan out four stacks of potatoes on some baking parchment on a baking tray so they are in a line. Drizzle with the olive oil and season with salt. Cook in the oven for 30 mins, turning once, until crispy. Can be prepared ahead and reheated.