In a small bowl, combine the vinegar and salt; slowly whisk in the walnut oil. Put the watercress in a salad bowl. Quarter and core the apple, slice it 1/8 inch thick, and then cut the slices crosswise. Add the apple to the salad bowl. Slice the endive heads on a sharp diagonal into 1/4-inch-wide strips, turning the heads as you slice and whittling down to the core. Add the endive to the salad, along with the walnuts. Toss the salad with the vinaigrette and arrange on plates. Crumble the Roquefort onto each serving, finish with a few grinds of black pepper, and serve.
Slicing the head on a sharp diagonal, turning as you go, gives you interesting endive shapes to toss into a salad. Stop cutting when you get to the tiniest center leaves and the core.
nutrition information (per serving):
Photo: Amy Albert and Scott Phillips