Melt the butter in a large pot and sauté the onion and celery in it for 10 minutes (do not let it brown).
Add the lentils, beef broth and spices and bring to a boil.
Cook over low heat for 30 to 35 minutes until the lentils are tender.
Season the soup with salt and pepper and serve.
Preparation Tip:
If the soup is too thick, simply dilute with a little water.