Combine broth, lemon juice, carrot, onion, celery, soup base, and pepper in Dutch oven and bring to boil over high heat. Reduce heat, cover partially and simmer until tender, about 20 minutes. Blend butter and flour in shallow bowl until smooth. Using back of fork, scrape butter mixture into hot soup a little at a time, stirring well after each addition. Simmer 10 minutes, stirring frequently. Meanwhile, beat yolks in large bowl of electric mixer on high speed until light and lemon colored. Reduce speed and gradually mix in some of hot soup. Return mixture to saucepan and cook until heated through. Stir in rice and chicken. Ladle soup into bowls. Garnish with lemon slices.