In a saucepan, heat the oil over medium-high heat. Saute the onion, carrot and celery until the onion is translucent, about 5 min. Stir in the thyme, bay leaf and barley, and cook 1 min.
Stir in the broth and bring to a simmer for 15 min. Remove the thyme. Let the broth cool completely. Add the sweet potato and lentils. NOTE: You can transfer to a container or resealable plastic bag to freeze for later use at this point.
Cook over medium heat, covered, until the sweet potato and lentils are tender, about 20 min. Garnish with fresh chopped parsley and shaved Parmesan, if desired.