1. Pour 1 1/2 cups boiling water over lentils in a small bowl; let stand 10 minutes.
2. Heat a Dutch oven over medium-high heat. Coat pan with cooking spray. Add carrot, celery, onion, and parsley to pan; sauté for 4 minutes. Add rhubarb, and sauté 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.
3. 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 splatters). Blend until smooth. Return pureed mixture to pan; stir in pepper.
4. Combine chopped dill and crème fraîche in a small bowl. Serve crème fraîche mixture on top of soup; garnish with dill sprigs, if desired.