Preheat the oven to 230°C, gas mark 8. Place the sweet potatoes on a nonstick baking tray and toss with 1½ tbsp olive oil and the harissa. Season and roast for 20-25 minutes, tossing now and then, until tender and burnished.
Meanwhile, mix together the tahini, yogurt, lemon juice and garlic until smooth. Set aside.
Transfer the cooked sweet potatoes to a platter, mix through most of the herbs and preserved lemon, then drizzle over the dressing, finishing with the remaining herbs and preserved lemon.
Cook’s tip
Tahini is made from pulped or ground sesame seeds and has a nutty flavour and creamy texture. It can be used in sweet and savoury recipes and is a key ingredient in houmous and halva. Stir through soups and stews at the end of cooking, or add it to dressings and sauces (adding a little to pan juices after cooking works well). It also works in baking and ice cream.
