Peel, wash and slice the potatoes. Cook in boiling salted water for about 15 minutes. Drain the tuna. Finely puree with yoghurt and lemon juice. Season to taste with salt and pepper.
Stir in the capers. Wash the parsley, dab dry, put something aside for garnishing. Cut the rest into fine strips. Drain the potatoes and let them drain. Roll up the turkey breast. Arrange potatoes and turkey breast, add sauce.
Sprinkle with parsley and garnish.