Pour 1 1/2 cups boiling water over lentils in a small bowl; let stand 10 minutes.
Heat a dutch oven over medium high heat. Coat pan with cooking spray. Add carrot, celery, onion, and parsley to pan; saute for 4 minutes. Add rhubarb and saute for 3 minutes. Drain lentils and add lentils to pan. Stir in chicken broth and salt; bring to a boil. Cover, reduce heat and simmer 35 minutes or until lentils are tender.
Remove from heat; let cool 5 minutes. Place 3 cups lentil mixture in a blender or food processor. Remove center piece of blender lid to allow steam to escape. secure blender lid on blender. Place a clean towel over opening in blender lid to avoid spatters. Blend until smooth. return pureed mixture to pan; stir in pepper.
Combine chopped dill and creme fraiche in a small bowl. Serve creme fraiche mixture on top of soup; garnish with dill sprigs.