1. Boil the potatoes until tender, then drain and allow to cool slightly.
2. Cut the potatoes in halves or quarters depending on their size.
3. Mix the potatoes with the mayonnaise/creme fraiche and spring onions.
4. Toast the sliced roll and then spread with grain mustard.
5. Fry the herring fillets in olive oil skin side down until the skin turns crispy. Turn over and cook flesh side for 1 minute.
6. Spoon the potato salad on to the rolls then top with herring fillet.