Wash, peel and cut celery into small pieces, bacon into wide strips. Fry both in oil, add lentils, roast briefly and douse wirh the soup.
Simmer about 15 minutes until the lentils are almost tender. Stir in creme fraiche, season to taste with salt, pepper and rosemary and simmer for another 3 minutes.
Boil pappardelle in salted water al dente and drain. Serve with the sauce.