Slice chicken breasts into bite-sized cubes and season generously with salt and pepper.
In a large Dutch oven, heat olive oil over medium heat. Working in batches as to not crowd the pan, brown chicken. Remove to a bowl and set aside.
Add butter to remaining oil and brownings and sauté onion, celery and carrots until onions are translucent.
Add garlic and Italian seasoning and sauté 1 minute. Add flour and cook 1 minute more.
Add chicken broth and bay leaves. Bring to a boil and then reduce heat and simmer for 10 minutes.
Add orzo and chicken and simmer 15 minutes more or until pasta is cooked through. The orzo will soak up quite a bit of broth, resulting in a thick soup. If desired, add additional chicken broth to adjust the consistency to your liking.
Remove from heat and add lemon juice and parsley. Taste and adjust any seasonings as necessary.