Drain tuna and capers separately. Set aside a few capers for garnishing. Blend the rest of the capers, tuna, salad mayonnaise and yoghurt with the hand blender. Season to taste with salt, cayenne pepper and vinegar. Put the tuna sauce in a cool place.
Wash and clean the tomatoes and cucumber and cut into large pieces. Peel onion and cut into medium-sized cubes. Wash parsley, dab dry, pluck and cut into small pieces. Mix the tomatoes, cucumber, onions and tuna sauce, season to taste if necessary and arrange on a plate with turkey slices. Garnish with remaining capers and parsley