Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper.
Line up the peppers on the baking sheet and bake on the middle rack for 15 minutes. Remove the peppers from the oven and pour out any excess moisture.
Sprinkle 1 tablespoon of the mozzarella into the bottom of each pepper cup. Top the mozzarella with 2 tablespoons of the pizza sauce. Evenly divide the pepperoni, Canadian bacon, mushrooms, olive and onion among the 8 pepper cups.
Top each pepper cup with 2 tablespoons of mozzarella cheese. Return the peppers to the oven and bake on the top rack for an additional 10 minutes. Remove the peppers from the oven and top each one with 1 tablespoon of Parmesan cheese and a sprinkle of Italian seasoning. Return the peppers to the oven and broil for 5 minutes.