In a kettle combine the chicken breasts with enough cold water to cover them by 1 inch and remove them. Bring the water to a boil, add salt and pepper to taste, and return the chicken to the kettle. Poach the chicken at a bare simmer for 18 minutes, remove the kettle from the heat, and let the chicken cool in the liquid for 30 minutes. Drain the chicken and let it stand until it is cool enough to be handled. Discard the skin and bones from the chicken, removing each breast half carefully from the bone in one piece, and chill the chicken, wrapped well in plastic wrap, for at least 6 hours or overnight. In a blender or food processor blend together the tuna, the mayonnaise, the yogurt, the anchovies, 1 tablespoon of the capers, the lemon juice, and salt and pepper to taste until the sauce is smooth, transfer the sauce to an airtight container, and chill it for at least 6 hours or overnight. Cut the chicken breasts diagonally into 1/4-inch-thick slices and transfer a breast to each of 6 dinner plates. Just before serving, stir the chopped basil and salt and pepper to taste into the sauce. Spoon some of the sauce over each breast, garnish the chicken with the additional capers, the basil sprigs, and the lemon slices, and serve it with the olives. Serves 6. Gourmet August 1992