Trim off the core end of each head of endive and slice it in half lengthwise. Cut out the cores, separate the leaves, and place 1 1/2 to 2 heads of endive on each plate. In a medium bowl, whisk together the vinegar, mustard, egg yolk, salt, and pepper. Slowly whisk in the olive oil to make an emulsion. Toss the pears with some vinaigrette and place on the endive. Drizzle the remaining vinaigrette over the endive leaves to moisten them. Crumble the Roquefort onto the endive. Sprinkle with walnuts and serve at room temperature.