How to Make It
In a large frying pan, toast the walnuts over moderately low heat, stirring frequently, until golden brown, about 5 minutes. Remove the nuts from the pan and chop them into fairly large pieces.
In the same pan, heat 1 tablespoon of the oil over moderate heat. Season the chicken breasts with 1/4 teaspoon each salt and pepper and add them to the pan. Cook the chicken until brown, about 5 minutes. Turn and cook until almost done, about 3 minutes longer. Cover the pan, remove from the heat, and let steam for 5 minutes. Remove the chicken breasts from the pan. When they are cool enough to handle, cut the chicken breasts into bite-size pieces.
Meanwhile, in a glass or stainless-steel bowl, whisk together the vinegar, sour cream, 1/8 teaspoon salt, and the remaining 1/4 teaspoon pepper. Add the remaining 5 tablespoons of oil slowly, whisking. Add the Roquefort and stir just to combine, leaving the dressing chunky.
In a large glass or stainless-steel bowl, toss the lettuce and the radicchio with half the dressing. Put the salad on plates. Top the salads with the chicken. Spoon the remaining dressing over the chicken and then sprinkle with the nuts.