Whisk together olive oil, vinegar and mustard in a bowl. Season with salt and pepper.
Preheat oven to 400 degrees F. (200 degrees C).
Peel pears, cut into quarters and core.
Cut into 10 to 12 thick slices each.
Place in a bowl and toss with the Port and olive oil. Arrange on a heavy baking sheet and roast for about 20 minutes or until slices are tender. Let cool. (Pears can be roasted up to a day ahead. Cover and Refrigerate).
Place walnuts on baking sheet. Toast in oven for 5 minutes. Remove stems from watercress. Wash and spin. Remove core from endives and cut into thick slices.
To serve, arrange watercress around edge of a large serving platter with endives in center.
Drizzle with dressing. Top with pear slices and sprinkle with Roquefort and walnuts. Serve immediately.