Roquefort, Pear & Watercress Salad

1 tablespoon white-wine vinegar, or Champagne vinegar

1 tablespoon walnut oil

1 tablespoon minced shallots

1 teaspoon Dijon mustard

Salt & freshly ground pepper, to taste

3 cups washed, dried and torn red leaf lettuce

3 cups washed and dried watercress leaves

1 ripe pear, preferably red, cored and thinly sliced

1 ounce Roquefort cheese, crumbled

1 tablespoon chopped toasted walnuts