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.