Place the dijon and vinegar in the bottom of the salad's serving bowl. Whisk with a fork until fully blended. Continue whisking as you pour the olive oil into the mix. Add salt and pepper to taste. Add radicchio to the bowl and toss with the dressing until coated. Pull the bleu cheese apart with finger into bite-sized pieced and sprinkle over salad. Sprinkle with walnuts. Serve.