Place both halves on a cookie sheet with foil over it for easy cleanup.
Spread 1 can of tomato paste on each half of bread.
Then place the pepperoni slices, and the onions, and peppers, and mushrooms. These can all be finely minced or coarsely chopped depending on personal preference. Minced does help to keep it all on the bread and spread easier though. I also like the texture of the minced pepperoni.
Finally top all the toppings with the sliced cheeses.
Place in the oven and bake until the cheese is melted - about 15 minutes.