In a large stockpot, place leftover whole roasted chicken (bones, meat, skin, everything) and cover with water. Boil chicken in water for at least 2 hours and up to 4 or 5 hours, making sure water covers the chicken throughout the time.
After at least 2 hours, remove chicken to a plate. Strain water (now chicken stock) through a sieve and reserve as chicken stock for the soup in a separate container until needed.
Place large stockpot back on stove over medium-low heat, and sauté the onion, carrots, celery, salt and pepper in the oil for 10-12 minutes.
Add the wine and turn up the heat to high until the liquid has completely reduced. Add 6 cups of chicken stock and bring to a boil.
Once boiling, add the orzo and cook for 7 minutes.
While orzo cooks, remove usable chicken from the whole roasted chicken, discarding bones/skin/inedible pieces, and place leftover chicken (ripped into bite size pieces) in the stockpot as the orzo boils.
Once the orzo is cooked after 7 minutes, add up to 2 more cups of stock if the soup has gotten too thick (usually a good idea because the soup thickens as it cools). Taste and add salt and pepper as needed (may need 1-2 tsps of salt).
For serving, scoop soup into bowls and top with chopped parsley and parmesan cheese. Enjoy!