In a small mixing bowl add the 1/4 cup flour to the 3/4 cup chicken stock and mix well to combine and then set aside.
Next in a large pot add in the olive oil and the onion and celery with a pinch of salt and cook about 6 minutes until tender and then add in bag of mixed vegetables, thyme, tarragon and some more salt and pepper and cook about another 5 minutes.
Nest add in the 4 cups of chicken stock, milk , flat leaf parsley and some more salt and black pepper and bring to a boil. Then add in the sweet potatoes and cook 15 minutes until the poatoes are tender.
Last, add in the chicken and the reserved "slurry" of chicken stock and flour and mix well. Continue to stir until the soup starts to thicken about 5 more minutes - check for seasoning and serve!