Preheat the oven to 130 °C.
Scrape the fish skin, rinse in cold water and dry gently with a paper towel.
Fillet the herring and remove the spine, but let the tail hang together. Remove all bones.
Brush mustard on the herring, cover with fresh dill and fold the fillets together.
Mix eggyolk and mustard on a plate and have rye flour on another.
Dredge the herring first in egg, then in the rye flour. Make sure it is completely covered in flour.
Pan-fry the fillets in butter on high heat until golden. Place in an oven-proof dish when cooked through.
Combine white wine vinegar, water, sugar and salt in a casserole, then the other ingredients for the brine.
Bring to a boil, pour over the herring and place the dish in the oven for approx. 10 minutes.
Cut the herring in pieces and place on slices of boiled potatoes with capelin roe, red onion and dill.