Heat oil in non-stick skillet over medium heat. Add onion and celery; cook until crisp-tender (about 3-5 minutes). Add garlic and saute, being careful not to burn.
Combine this with remaining ingredients in a large Crock-Pot. Cook on Low 4 hours or on High 2 hours, or until lentils are tender.
Garnish with shredded mozzarella cheese and finely chopped roasted red peppers. You could also serve over or stir in your favorite pasta or rice.
NOTE: For more crisp-tender texture, add the frozen veggies during last 30-60 minutes of cooking time. My kids don't like the taste of celery, so I chop it very finely so that it "disappears" in the soup.