Heat oil in a large saucepan or soup pot. Add leeks or onion and celery; sauté over medium heat until tender, stirring occasionally, about 5-10 minutes. Stir in garlic and curry powder and sauté, stirring often, for 2 minutes. Add lentils, water, bay leaf, salt, carrots and potatoes (optional). Bring to a boil; reduce heat, cover and simmer soup for 30 minutes or until lentils are tender. Stir in tomatoes and simmer for 5 minutes. Remove bay leaf. Stir about 1 tablespoon sour cream into each bowl of soup at serving time.