For dressing, peel peach. Coarsely chop. Place in a blender or food processor. Add vinegar and salt. Whirl until puréed, then add oil and whirl. Dressing will be quite thick. It will keep well, covered and refrigerated, up to 3 days.
Just before serving, place spinach in a very large bowl. Slice prosciutto into thin strips. Peel peaches, if you wish. Slice in half, then thinly slice into wedges. Thinly slice onions. Add all to spinach. Drizzle with dressing and sprinkle with nuts. Toss to evenly mix.