This is a very simple, throw together at the last minute kind of meal. There are only three basic ingredients, and then you can add whatever extras you happen to have in the house.
The three "must have" ingredients are foods I usually have on hand every day: bagels, sauce, and mozzarella cheese. Most of the time I do have pepperoni, because a local grocery store puts small packs on sale often, and I usually stock up.
These are great for parties, too, especially if you use the mini bagels.