Toss the walnuts with just enough vinaigrette to coat. Toss the endive with most of the remaining vinaigrette and arrange the leaves on a salad plate. Top with the apple slices and drizzle with a little vinaigrette. Sprinkle with the walnuts and serve at once.
To make Walnut Vinaigrette, whisk the oils and vinegars together, then season to taste with salt and pepper. This vinaigrette is also excellent with a salad of beets, oranges, onions, and watercress, a salad of romaine, butter lettuce, and Gruyère or Gorgonzola cheese, and on grilled leeks.