Slit prunes lengthwise down 1 side and remove pits. Squeeze prunes from ends to open slits, and fill equally with cheese. Press prunes to close around cheese. Wrap a piece of prosciutto around each prune, securing with a toothpick. Place prunes in a 10- by 15-inch pan.
Bake in a 450° oven until proscuitto is well browned, about 10 minutes (about 6 minutes in a convection oven); after about 4 minutes, use a wide spatula to ease prunes loose from pan and turn over.
Meanwhile, in a bowl, combine oil and vinegar. Add spinach and mix gently, seasoning to taste with salt and pepper.
Spoon spinach mixture onto plates. Remove toothpicks from prunes and set an equal number on each salad.