In large saucepan, heat oil over medium heat; fry onion, garlic, carrot, celery, thyme, salt and pepper until softened, about 5 minutes.
Add stock, 2 cups water, lentils and barley; bring to a boil. Reduce heat, cover and simmer until barley and lentils are tender, about 40 minutes. Stir in chopped parsley.
Make ahead: Let cool for 30 minutes;refrigerate, uncovered, until cold. Cover and refrigerate up to 2 days