Sauté the celery, carrots, and onions in a large soup pot with the butter and oil over medium-high heat for 5-7 minutes.
Stir in the garlic and cook for about 30 seconds, then add the flour and cook for another minute or so.
Pour in the chicken broth and stir until the flour has dissolved, then add the Italian seasoning and chicken. Bring the soup to a boil.
Cover the soup and reduce the heat and simmer for 15 minutes.
Stir in the orzo and cook uncovered for another 10 minutes or until the orzo is cooked through, stirring occasionally.
Remove the chicken from the pot when it reaches 160˚F and cut it up.
Remove the pot from heat.
Add chicken back to pot.
Add the lemon juice, parsley, and season the soup with salt & pepper as needed.