Preheat oven to 425 degrees F. Spray a 9 x 13-inch baking dish with cooking spray.
In the prepared baking dish (or in a separate bowl), stir together uncooked pasta, marinara sauce, water, about ¾ of the package of pepperoni, and salt. Cover tightly with aluminum foil and bake for 25 minutes.
Remove dish from oven, take off cover, and give the pasta a stir. At this point it should be almost done, but still a bit firm ("al dente").
Sprinkle mozzarella over the top and place remaining pepperoni slices on top of the cheese. Return the dish to the oven and bake, uncovered, for about 5-10 more minutes (or until cheese is melted and pasta is tender).
Garnish with optional grated Parmesan cheese and fresh herbs, if desired
