Whisk creme fraiche with yogurt and vinegar. Season with salt and pepper.
Rinse, trim and finely grate radish. Mix with yogurt-cream. Peel, quarter and remove cores from apples. Finely grate apple quarters into the yogurt-cream.
Cut herring fillets crosswise into finger-width strips, then cut into cubes. Peel, halve and finely chop onion. Finely dice pickled cucumber. Mix herring with onion, pickle and chives. Season with salt and pepper. Refrigerate and let marinate for 1 hour before serving. Divide salad among small bowls and garnish with trout roe. To serve, sprinkle with chives and top with onion slices.