Preheat the oven to 200°C/fan 180°C/gas mark 6. Lay the pieces of carrot and sweet potato in a large, high sided roasting tray with the onion. Add in the remaining ingredients and mix well.
Season, cover with tin foil and cook for 30 minutes, then stir well and cook uncovered for a further 40-45 minutes, adding a splash of water occasionally if it starts to dry up too much. It should be soft and caramelised once finished.