Preheat the broiler. Arrange the prosciutto in a single layer on a foil-covered baking sheet and broil until crisp, 3 to 4 minutes. Set aside.
Toss the mushrooms, celery and onion with the EVOO and lemon juice; season with salt and pepper. Arrange on a platter and top with the arugula and parsley. Drizzle with truffle oil, if using. Break up the crispy prosciutto and scatter over the salad.