Roughly chop onions, carrots, celery and leek.
Generously coat base of large pot with oil and place over medium heat.
Add chopped vegetables, stir briefly to coat with oil, cover with a lid, turn down to a low heat and sweat for 15 minutes stirring occasionally.
Add two litres of water and the red lentils. Stir carefully to ensure that the lentils do not stick to the base of the pot.
Bring to a boil and then turn the heat down.
Cover with a lid and simmer for 30 minutes or until the lentils are cooked and the carrots are soft.
Remove the pot from the heat, add the fresh basil, reserving some sprigs for garnish.
Blend until smooth using a stick (immersion) blender.
Add water if it’s too thick.
Season with salt and pepper.
Bring back to a simmer and serve.
Garnish each bowl with a sprig of basil.