How to Make It
Preheat the oven to 350°. Spread the walnuts in a pie plate and toast for about 8 minutes, or until fragrant and golden. Let cool.
In a large bowl, combine the shallot with the vinegar and let stand for 5 minutes. Whisk in the mustard, then in a thin stream whisk in the oil. Season generously with salt and pepper. Add the endives, radicchio, arugula, parsley and walnuts and toss to coat. Top with the prosciutto and serve.