Preheat the oven to 400 degrees.
Bring a large pot of salted water to a boil.
Cook penne according to package directions.
Heat the olive oil in a large pan over medium heat.
While the pasta cooks, prepare the meat and vegetable mixture.
Add the onion, mushrooms and bell pepper to the pan. Cook for 4-5 minutes or until vegetables are just tender. Season the vegetables to taste with salt and pepper.
Add the sausage to the pan. Cook, breaking apart with a spoon, until meat is cooked through, about 5-7 minutes.
Stir the olives into the sausage mixture.
Drain the penne pasta and pour it into a 9"x13" pan that's been coated with cooking spray.
Add the sausage mixture and marinara sauce to the pan and toss gently to coat the pasta.
Sprinkle the top of the pasta with the cheese and arrange pepperoni over the top.
Bake for 20 minutes, or until cheese is melted and just starting to brown.
Sprinkle with parsley and serve.