Combine water, white wine vinegar, salt and sugar until everything has dissolved.
Pour the brine over the herring fillets, cover with a lid and set in the refrigerator over night. If you are using spice-salted or salted herring fillets they should be left in cold water for approx. 30 minutes instead of leaving it in the brine over night.
Let the herring drain on a paper towel and remove all visible bones. Cut the herring in 1-2 cm wide pieces.
Whisk egg yolk and add a mix of sunflower oil and olive oil while continuously whisking until it has a consistency of mayonnaise.
Add crème fraîche, mustard, vendace roe, finely chopped tarragon and chives to the mayonnaise and mix well. Add the herring.
Cut rye bread in thin slices and cut away the crust. Cut each slice in 4 pieces and cover with the herring.
Garnish with thin slices of leek and fresh chives.