Cut the loaf of bread in half crosswise, then cut each half lengthwise into 1-inch thick slices. Lay the slices in a single layer on baking sheets. Generously drizzle the top side of each slice with olive oil and season with salt and pepper to taste. Sprinkle the Parmesan over the slices. Cook in the oven until golden brown, about 10 to 12 minutes. Set aside.
Marinate the onions in the vinegar for 5 minutes. While the onion is marinating, peel, seed and dice the melon and place in a bowl. Add the diced melon to the onions, season with salt and stir to blend together. Add the minced parsley and stir to combine.
Wrap the end of each crostini with a piece of prosciutto. Arrange on a platter and spoon a tablespoon of melon salsa on each and serve. Serve remaining salsa on the side.