Mix sour cream, yoghurt and wasabi. Season with some salt and pepper. Clean and wash spring onions and cut into fine rings. Wash parsley, shake dry. Put something aside for garnishing, cut the rest finely.
Finely dice the tuna. Mix with spring onions, parsley and 1-2 tbsp. oil. Season to taste with lime juice, salt and pepper.
Place tartar one after the other in a lightly oiled starter ring (8 cm Ø) on four plates and press down lightly, carefully pull off starter ring (or form 4 flat taler with moistened hands).
Place a blob of yoghurt dip on each and garnish with parsley leaves. Sprinkle with pepper. Garnish with lime wedges if desired. Serve with the rest of the yoghurt dip. Roasted baguette tastes good with it.