Heat a generous drizzle of oil in a large saucepan over medium heat. Add the onion and cook till softened, then add the carrot and lentils and stir to coat in the oil, cooking and stirring for a couple of minutes.
Add the water and tomato puree and bring to the boil. Cover, lower heat and simmer till soup is thick and lentils are tender, 20 to 30 minutes.
Remove soup from heat and cool slightly; season to taste. Add the parsley and puree the soup using a hand held blender. Return to the heat; warm through and serve.