In a large pot, combine chicken broth, fresh lemon juice, shredded carrots, finely chopped onion, and finely chopped celery. Add chicken soup base and ground white pepper, then bring the mixture to a boil over medium-high heat. Once boiling, reduce to a simmer and cook for 15-20 minutes until the vegetables are tender.

In a small bowl, mix margarine with all-purpose flour until smooth, creating a paste. Gradually stir this mixture into the soup, stirring constantly for 8-10 minutes until thickened.

In a separate bowl, beat the egg yolks until pale and creamy. Whisk in about a cup of hot soup to temper the yolks, then pour this mixture back into the pot, stirring gently. Cook on low heat for a few minutes without boiling.

Stir in the cooked white rice and diced cooked chicken. Increase the heat slightly and simmer for an additional 5 minutes until heated through.

Ladle the warm soup into bowls and garnish with fresh lemon slices and optional herbs. Serve immediately.