Rinse the matjes herring fillets under cold water and pat dry with paper towels. Cut the fillets into bite-sized pieces.
In a small saucepan, sauté the chopped onion over medium heat until softened and translucent, about 5 minutes.
Add the heavy cream, sour cream, Dijon mustard, honey, and white wine vinegar to the saucepan, stirring until well combined.
Reduce the heat to low and simmer the sauce for 5-10 minutes, stirring occasionally, until it thickens slightly.
Season the cream sauce with salt and pepper to taste.
Let the sauce cool down
Add the matjes herring pieces to the saucepan, gently stirring to coat the herring with the cream sauce.
Cover the bowl with cling wrap and refrigerate for at least 12 hours.
Enjoy cold with fresh boiled hot potatoes