In a mixing bowl, combine the vinegar, shallot, garlic, mustard, salt, and pepper and whisk to combine. While continuously whisking, add the oil in a slow, steady stream until completely incorporated. Whisk in the herbs, and set aside while you prepare the salad.
In a large bowl, combine the arugula and red onion. Drizzle in 1/4 cup of the vinaigrette and toss to combine. Add more vinaigrette to taste, if desired, and season lightly with salt and pepper. Toss gently to combine.
Arrange the cantaloupe wedges on a large serving plate, top with the arugula salad and the prosciutto. Serve immediately.
Helpful Hint: Any unused vinaigrette can be stored in a nonreactive, airtight container in the refrigerator for up to 2 days.
Yield: 4 to 6 servings