Preheat oven to 200°C/400°F/Gas Mark 5. Line a roasting dish with baking paper. Cut yams lengthways and toss with olive oil, and some salt and pepper. Roast until almost cooked, 25–30 minutes. Add maple syrup and pecans to yams, mix them together and return to the oven. Roast until yams begin to lightly caramelise. Gently toss again and serve straight from the oven.