Kids love these pizza-topped skins because they end up with two of their favorite dishes: mashed potatoes one day, potato skins the next.
Heat your oven to 400°. Scrub the potatoes, poke them with a paring knife, then bake them for 50 to 60 minutes, until they're tender at the center. Cool the potatoes briefly, then halve them lengthwise and scoop out most of the flesh (use it for mashed potatoes), leaving a thin potato wall. Cool the skins. If you're not preparing the pizza the same day, refrigerate them.
To continue preparing the pizza the same day, heat your oven to 400°. Arrange the skins on a well-greased baking sheet, radiating out like the spokes of a wheel, as shown. Cut some skins in half the long way and tuck them into the gaps between the whole skins. Lightly salt and pepper the insides of the skins.
Spoon the sauce into the skins, dividing it equally. Sprinkle one or more toppings over the skins, if desired, then cover them evenly with the Parmesan cheese. Sprinkle with the mozzarella. Bake on the center oven rack for 15 minutes. Cool for several minutes, then serve. Makes 5 to 6 servings.