I guess the Nordic like to eat brined matjes w chopped onion plain as that.
I do have a good quick sandwich for herring:
1 poppy seed bagel sliced preferably toasted
2 -3 slices ripe tomatoes
Alfalfa or onion sprouts
Two fillets of brined matjes
Some chopped red onion
Spread mayonnaise lightly on the bagel and assemble!