Steam the onions in the butter until translucent, then add the lentils and cook for 1 minute. Add the bouillon and let everything simmer on a low flame for approx. 20 minutes.
In the meantime, prepare the celery and cut into pieces approx. 1/2 cm wide. The leaves can be used as a garnish.
After the 20 min. Cooking time, add the celery to the soup and let everything simmer again for approx. 8-10 min.
Stir in the creme frâiche, garnish with the celery leaves, serve.