Combine chicken broth, lemon juice, carrots, onions, celery, soup base, and white pepper in a large pot. Bring to a boil over high heat, then reduce heat and simmer for 15 to 20 minutes, or until the vegetables are tender.
Blend butter and flour in a small bowl; gradually stir into soup mixture. Simmer, stirring frequently, for 8 to 10 minutes.
Meanwhile, beat egg yolks in a bowl until light in color. Gradually whisk in some hot soup, using a ladle to pour in a thin stream while whisking the egg yolks vigorously. Add egg mixture to the pot in same manner, and heat through.
Add rice and chicken; cook until warmed through. Ladle hot soup into bowls and garnish with lemon slices.
