1.To make dressing, whisk vinegar, mustard, oil and sugar in medium bowl. Combine pork and 2 tablespoons of the dressing in large bowl. Reserve remaining dressing.
2.Make fennel apple relish. Combine apple, onion and fennel in bowl with reserved dressing.
3.Drain pork; reserve marinade. Cook pork on heated oiled barbecue (or grill or grill plate) until browned both sides and cooked as desired, brushing with marinade occasionally.
4.Meanwhile, boil, steam or microwave potatoes until tender; drain. Mash half the potatoes with sour cream and butter in large bowl until smooth; stir in herbs. Using back of fork, roughly crush remaining potatoes until skins burst and flesh is just crushed; stir into herbed mash.
5.Serve pork with relish and herbed potato.
Pork can be marinated overnight, covered, in the refrigerator.Note