Bring 600ml water to the boil in a deep saucepan. Turn the heat up and keep the water gently bubbling. Place the sweet potato in the water and boil for 5-7 minutes, until soft.
Heat the olive oil in a non-stick frying pan over a medium-low heat. Add the onion and garlic and sauté for 3-4 minutes. Stir in the tomatoes and the mixed herbs.