In the meantime, cook the couscous according to the instructions. Usually this is just a little salted and poured with boiling water. Set aside and let it steep.
When the lentils are soft, add the canned tomatoes, season with salt and lemon juice. Now add the feta (set aside for garnish) and puree the soup.
Arrange in plates and add the couscous and the rest of the feta.