Place 1 chicken breast half, boned side up, between 2 pieces of plastic wrap or waxed paper. Working from center, gently pound chicken with flat side of meat mallet or rolling pin until about 1/4 inch thick; remove wrap. Repeat with remaining chicken breast half.
Place flour, 1/3 cup dressing and pecans in 3 separate small flat dishes. Coat each chicken breast half with flour. Dip in dressing to coat well. Dip in pecans, coating both sides.
Melt butter in medium nonstick skillet over medium-low heat. Add chicken; cook 2 to 3 minutes on each side or until chicken is fork-tender and juices run clear. Remove chicken from skillet; place on serving platter. Cover to keep warm.
To drippings in skillet, add 2 tablespoons dressing and sour cream; cook and stir until mixture boils and thickens. Pour over chicken. Sprinkle with onions.