Sprinkle salt & ground pepper on both sides of chicken breasts.
Melt butter in a large frying pan, over low heat. Add the chicken, cooking slowly to brown on all sides. Add the stock and simmer for 5 - 6 minutes. Add the celery flakes, onion powder, and thyme; for 5 minutes. Remove chicken to platter and set aside.
Add the flour and whisk until smooth. Add the mushrooms and wine. Return chicken to pan and cover; simmer for 10 minutes, or until the chicken is tender.