A day ahead: Remove the skin from the herring fillets and check against bones. Salt the fish lightly and place them in the freezer for at least 24 hours.
On the day you serve the fish: Take the fillets out of the freezer, slice them thinly and arrange them flatly on a platter. Prepare the red onion and the fennel, saving the top of the fennel for later. Chop the fennel and onion finely. Crush the juniper and caraway seeds and add them in a pot to the vinegar, sugar and rapeweed oil, giving the vegetable marinade a quick boil. Let the liquid cool.
Spread the warm marinade over the herring and let them absorb it for about 10 minutes. Chop up the fennel top and sprinkle it over the herring before serving.