Chop the onion, carrots, leek and celery into small pieces.
Sauté the onion in the olive oil. Add the vegetables and fry for a few minutes. Add tomatoes, lentils, bay leaf and sage (tie the sage together so that you can fish it out later). Pour approx. 1 liter of water.
Let the lid simmer gently for about 1 hour with the lid placed at an angle. Season with salt and pepper.
As an alternative to the leek, you can also add 2 stalks of white celery.