Debone and dry the herring fillets. Cut the fillets into small cubes.
Cut the individual grapes in half. Peel the boiled potato and dice it as well.
Whisk the mayonnaise with sour cream, vinegar, sugar, pepper and salt. Peel and finely chop the onion and stir it into the sauce.
Fold the herrings, grape pieces and potatoes into the sour cream mixture.
Leave to infuse for approx. 1 hour and then serve.