In food processor, finely chop pecans, cornstarch, thyme, salt cayenne and dry mustard.
Whirl in parsley.
Transfer to shallow bowl;set aside.
In separate shallow bowl, beat egg.
Dip each chicken breast into egg, then into pecan mixture, coating both sides well.
(Make ahead: transfer to platter and cover loosley with plastic wrap; refrigerate for up to 2 hours.) In large nonstick frypan, heat oil over medium heat; cook chicken, turning once, for 15 to 20 minutes or until no longer pink inside.
Sauce: Meanwhile, in small bowl, whisk together sour cream, Dijon mustard, sugar and salt.