Heat oil in a large stockpot; saute onion, garlic, carrots and celery; cook about 5 minutes until onions are softened. Transfer vegetables to slow cooker. Add water or stock, lentils, tomato sauce, tomato paste, bay leaves, salt and pepper.
Cook on low for 8-9 hours until lentils are tender. Remove bay leaves. Stir in vinegar just before serving.
2 Freestyle points per serving