Preheat oven to 350°F and grease a 9x13" casserole dish with cooking spray.
Cook rotini according to package instructions, to al dente, then drain.
While the rotini is cooking, heat the vegetable oil in a large saucepan over medium-high heat.
Saute the diced green peppers and onions for 3-5 minutes, or until softened.
Add the Italian sausage and cook thoroughly; remove from heat.
Add the cooked pasta, both jars of pasta sauce, garlic powder, mushrooms, pepperoni slices, half of the mozzarella cheese and the parmesan cheese.
Stir well, then pour into the prepared casserole dish.
Bake for 30 minutes.
Remove from the oven and sprinkle the remaining mozzarella cheese on top; bake an additional 5-10 minutes, or until the cheese is melted.
Enjoy warm.