Carefully wash the arugula in several changes of water, being aware of the dense sand usually found on this green. Trim off and remove the stems. Dry the leaves in a salad spinner.
Slice the smoked mozzarella into thin 1/8 inch slices. Slice the tomatoes as neatly and evenly as the cheese, 1/8 inch thick.
Brush the bread with 2 tablespoons extra virgin oil and grill or toast under a broiler until crisp and golden. Encircle the outside edge of a serving platter with the crostini. Lay the clean arugula out in the center of the platter as a base for the salad.
Combine the vinegar with the mustard in a small stainless steel bowl. Whisk together briskly while drizzling in the olive oil. Add the diced onion and combine.
Carefully layer the slices of mozzarella, tomato, prosciutto and basil leaves around the greens. Drizzle vinaigrette over and around, sprinkle pine nuts and serve.