In a Dutch oven or large stockpot, heat olive oil over medium high heat until hot but not smoking. Add onion, garlic, carrots, and celery. Saute for about 10 minutes or until onion is translucent and vegetables are tender crisp.
Add lentils, tomato sauce, water, vegetable broth and bay leaves. Bring mixture to a boil. Reduce heat; cover and let simmer for about 25 to 35 minutes or until vegetables are soft but no mushy.
Add cooked meat and salt and pepper. Let cook for another 10 minutes or until meat is heated through.
Ladle soup into bowls and top with lemon slices. Place a cruet of vinegar on the table and invite diners to add a splash of vinegar to their soup.