Clip the dorsal fin off the herring fillets and rinse them thoroughly. Pat them dry on the flesh side with a piece of kitchen paper.

Put the rye flour in a deep plate and season it with a little salt and pepper.

Toss the herring fillets in the rye flour.

Heat a pan with the butter.

Fold the herring fillets lengthwise and fry them in butter for 3–5 minutes on each side. Remove them from the pan and set them aside.

Put vinegar, sugar, water, and peppercorns in a saucepan. Bring the mixture to a brief boil and let it cool slightly.

Layer the herring fillets and onion rings in a container and pour the marinade over them.

Cover the container and refrigerate until the herring is ready to be eaten.