Heat the oven to 350°F/175°C. Toss the walnuts in the walnut oil and sprinkle with salt and pepper. Spread them on a baking sheet and bake for 5 minutes or until toasted. Leave to cool. Fry the croûtes.
Arrange the lettuce leaves on four large plates. Separate the watercress into four bunches and arrange on top of the lettuce. Crumble the Roquefort, toss with t