Preheat the oven to 180°C, 160°C fan, Gas 4. Cut the sweet potato lengthways into 8 wedges, toss in the oil and roast until tender (for about 40 minutes). Allow to cool then chill overnight.
Peel 2 cooked potato wedges and cut into chunks. Mix with 100g of the tomatoes, 50g of the salad leaves and the cranberries. Whisk the oil, balsamic vinegar and orange juice together and put in a container with a tight-fitting lid. Drizzle over the salad before serving.