Put the sea buckthorn berries, sugar, turmeric and water in a saucepan and bring to the boil. Let it boil briskly for 3-4 minutes, then turn off the heat. The marinade will thicken as it cools.
Season the marinade with salt and pepper and leave it to cool to room temperature before adding the herring.
Cut the herring fillets into bite-sized pieces and pat them dry with a little kitchen paper.
Fold them into the sea buckthorn marinade and serve the herring topped with crème fraîche, onion rings and fresh dill.