Preheat the oven to 190°C (gas 5). Use 10g of the butter to grease a baking dish, in which the rolled up fish fillets will fit snugly side by side.
Cook the potatoes in boiling water for 15 minutes or until almost tender, but still firm. Drain and cool, then peel off the skins and slice them thinly.
Rinse the herring fillets and pat dry on kitchen paper. Place them, skinned-side up, on a board. Mix the remaining butter and mustard together. Reserve a third of the mustard butter, then thinly spread the rest over the fillets. Roll them up from the tail end.
Peel, quarter, core and thinly slice the apples. Put a layer of potato slices in the bottom of the baking dish, topped by a layer of apple slices. Mix together the stock and cider and season with a little salt and pepper. Pour a little over the apple slices to just cover them and stop them from discolouring.
Continue layering the potato and apple slices until you have used about two-thirds, adding a little liquid after each apple layer. Place the rolled herring fillets on top, seam-side down, and add the rest of the potato and apples, finishing with a layer of potatoes.
Pour over the remaining stock and cider – it should just cover the potato topping, so add a little more stock if necessary. Dot the reserved mustard butter over the top and bake for 40 minutes or until the potatoes and apples are tender and the fish is cooked. Serve hot, sprinkled with fresh chives.
Cook some roasted courgettes or braised cabbage at the same time.