Pre-heat the oven to 220 degrees. Place the sweet potato on a baking tray, toss with olive oil and sprinkle with salt and pepper. Roast for 20-25 minutes until soft. Remove from the oven and leave to cool Whilst the potato is roasting mince the garlic and combine, the tahini, lemon juice and olive oil with a tablespoon of water. Whisk until smooth. When the potatoes have cooled, combine with the red onion and chickpeas then drizzle with the dressing