Strip the leaves from the thyme branches and chop them coarsely. Heat the oil and cook the onion, thyme & carrots for about 4 minutes until just soft.
Add the garlic, chilli and mustard seeds and cook for a further couple of minutes. Stir in the tomatoes, lentils and stock and bring to the boil. Reduce the heat, cover and simmer gently for 30 minutes.
Season with the pinch of sugar, salt and pepper. Blend until smooth or leave chunky.