Preheat the oven to 200°C/400°F/ Gas Mark 5. Using a knife, thinly slice the sweet potato and beetroot into wafer-thin slices. Spread out onto paper towel to remove any excess moisture.
Place the slices into a large bowl and sprinkle with the oil and seasoning – toss to coat evenly. Arrange the slices in a single layer on baking trays and roast for 20 minutes, turning them after 10 minutes. Remove from the oven when crisp and golden and spread out on a paper towel until cool.