Preheat oven to 180C. Peel and chop all vegetables. In a large saucepan, add the onion, garlic and tomato and saute with water. When fragrant, add the vegetable stock, carrot, lentils and herbs. Simmer for 10 minutes or until vegetables have cooked through. Add salt and pepper to taste.
For the topping, peel and chop the potato and boil in a saucepan for around 15 minutes or until soft. Drain and add to a bowl with the milk and desired seasonings. Use a whizz stick to puree and set aside.
Pour lentil mixture into a casserole dish. Spread the sweet potato on top as desired. Bake for 10 minutes or until the top of the potato has turned golden brown.
Remove and let cool for 5 minutes. Enjoy!