Sauté the celery, carrots, and onions in a large soup pot with the butter and oil over medium-high heat for 5-7 minutes.
Pour in chicken stock and bring to a gentle boil. Add orzo and cook 8–10 minutes until tender.
Stir in shredded chicken and heavy cream. Reduce heat to low.
Stir in fresh lemon juice. Season with salt and pepper.
Cover the soup (lid slightly open), and reduce the heat and simmer for 15 minutes.
Ladle into bowls and garnish with fresh herbs, if desired.