Pound each chicken breast (between wax paper) with rolling pin until about 1/4" thick.
Place flour, 2/3 cup salad dressing, and pecans in three separate dishes. Coat chicken with flour then dressing, then pecans, pressing to coat well.
Melt butter in heavy, non stick skillet, and cook coated chicken 2-3 minutes on each side until chicken is thoroughly cooked. Watch carefully, as the pecans can burn easily! Remove chicken from skillet and cover to keep warm.
To drippings remaining in skillet, add 1/4 cup salad dressing, sour cream, pepper, and thyme and heat until mixture boils. Pour over chicken to serve.