Shell eggs, then cut in half lengthwise. Transfer yolks to small bowl and mash with fork. Mix in sour cream, mayonnaise, and mustard. Stir in lemon peel, lemon juice, and cayenne pepper. Season generously with salt and pepper.
Spoon yolk mixture into whites. Sprinkle generously with chopped parsley or thyme. (Can be made 4 hours ahead. Cover loosely and refrigerate.)
